Environmental conditions such as atmospheric pressure, temperature, and humidity affect the output of engines. Therefore a correction of the output to the standard atmospheric condition is made for proper description of engine performance, Each country has prescribed its own procedure for output correction, but recently the ISO standard is generally being followed. The authors have been working to collect standard data related to each kind of engine which will form the basis of the correcting formula. In this study they have examined the problem points of the influence of atmospheric conditions on diesel engine performance and those of the procedure for output correction, based on an experiment using a swirl chamber type naturally aspirated diesel engine. The engine tests were run keeping the fuel injection pump in a so called “fixed rack position”. One problem in performing these kind of tests is that when the environmental conditions are changed, the fuel injection quantity cannot be kept constant. However, the authors proposed an appropriate method of correction for this problem.