This paper inverstigates a new way of conical spray for medium speed D. I. diesel engine, with which three different construction injectors were used. The feature of conical spray and fuel-air mixture formation were observed by means of schlieren photograph technique. The main result is that the cone top angle of conical injector has influence on formation of fuel-air mixture and performance of engine. The results of test on a single-cylinder engine show the premixed combustion phase was possessed of a large proportions of the whole combustion period, which was become a leading feature.The increasing interest in study of diesel engine combustion is caused by achieving even more stringent emission standards and greatly improving the fuel economy. From present status of this research the traditional combustion system which with orifice nozzel has already exposed some inherent drawbacks. First, the fuel delivered from the nozzel is disintegrated into droplets of different sizes and concentrations in spray. It is very difficult to get a good homogeneous air-fuel mixture. Also the uneven distribution of the fuel whithin the combustion space has an adverse effect on the mechanisms of combustion and emission formation. Secondly, due to the heterogeneity of the mixture concentration the ignition delay is uncontrolled and take fire always is early at B. T. D. C. As a consequence of those an over-high maximum combustion pressure or a prolonged combustion duration is to be unavoidable. Finally, autoignition occurs easily at several locations in the combustion chamber where there is a combustible mixture because of the heterogeneity, meanwhile, in some other locations the fuel may still be in the liquid phase. Under most engine operating conditions, ignition starts while some portion of the fuel has not yet been injected. It is clearly that a large quantities of fuel inject into the combustion chamber when the combustion started, so that the diffusive combustion is become dominant. As we know that as compared with premixed combustion a combustion performance of the former is less perfect than one of the latter.The conical spray combution system is a new way of fuel-air mixing and combustion process for D. I. Diesel. It has been developed by our institute for many years. Owing to the special constructure of injector it has achieved an uniform peripheral distribution and good atomization quality of spray as well as the higher fuel injection rate. For this reason, the combustion characteristic and engine performance will possess new feature.This paper presents some results of study about conical spray injection and its combustion. Schlieren photograph is applied to survey the development of conical spray. It was applied in a single engine which was modified and running by motoring. In order to get ideal optical access a rectangular cross-section combustion chamber are adopted and two quarts glasses are mounted in the lateral side of combustion chamber as observation window. The schlieren system used is of the Z type in configuration and it consists of two spherical mirrors and two flat mirrors, a point sourse and a knife edge. The device and optical path are schemed as shown in Fig. 1. A synchronization of the device and high-speed camera as well as other measuring instruments were controlled by a micro - computer with an amplified circuit. When taking schlieren pictures the device was driven by a direct current motor and its speed is 750 rpm. The main parameters of this system and device are given in Table 1 and Table 2 respectively The performance test engine is the same as photo-modified one (Fig. 2), but it is a baseline. The rated output is 26 KW. Engine was operated according to load characteristic. Data acquisition and post-experiment analysis were used by a AVL 657 Digital Analyzer. Exhaust gas emission was measured in FP6 emission analyzer.Three typical nozzels were adopted to survey the characteristic of conical spray and combustion. They have different specifications. The diameter of nozzel and injection angle are φ1.5 (mm) * 110° (injector A) φ2 (mm)* 110° (injector B) and φ2 (mm) * 130° (injector C) respectively. Fig. 3 shows the construction of nozzel of conical spray injector. In order to form a true estimation of the results of experiment the conditions of test must have identification as follows. The needle valve openning pressure is 24 MPa. The fuel injection timing is 24 CA before T.D.C.