The purpose of this research is to investigate the combustion characteristics in the diesel engine, the effect of engine operating parameters on the exhaust emissions, and the relationship between exhaust emissions and the engine deposit formation and composition. A YAMAHA ME200F 3-cylinder DI diesel engine is used in this study. A complex procedure is developed in this research to investigate the deposit formation in the piston and cylinder head and the deposit's physical factor (weight, color, etc.) and chemical composition analyses are also performed.In this study, a complete set of gas analyzer (including NOx, total HC, COx, smoke number, and smoke opacity) is used. After continuous operation of engine under different engine load and speed, the cylinder head is dissembled to allow inspection of deposits on the piston and cylinder head. The deposit is separate into soluble and insoluble fractions for weight analysis. The deposit sampling metal plate is removed from the piston and cylinder head after 25 hours continuous operation for SEM (Scanning Electronic Microscope) analysis on the soot particle and deposit crystallization. Also the deposit's element composition is determined by using EDS (Energy Dispersive Spectrometer) method.Results show that increased engine speed will shorten the NOx formation duration and thus reduce the NOx emission level; however, increased engine load will promote cylinder charge temperature and thus increase NOx emission concentration. The hydrocarbon (HC) emission is reduced after the increase of engine speed and/or load. This is because increased engine speed will increase the turbulence level (promote the HC/O2 mixing) in the cylinder while increased engine load will increase average cylinder charge temperature; thus, the HC oxidation rate is raised and the HC emission is reduced. The smoke pollution level increases with the increase of engine speed and/or engine load which is in agreement with the raise of exhaust temperature. In the mean time, the add up of HC and particulate concentration reaches a constant level also reveal the hypothesis that unburned hydrocarbon is the precursor of the particulate. The weight of deposits in the engine is first increased and then decreased when the engine speed and/or load are increased. This suggests that the engine deposits can be oxidized under high temperature cylinder charge when the engine speed and/or load is increased upon a critical value. Also the SEM result shows that when the engine speed is decreased or the engine load is increased, the soot particle size is increased; however, the average structure diameter of the deposit crystal is decreased. The EDS inspection shows that the atomic mole percentage of carbon and oxygen in the deposit as well as iron, chromium, and nickel which is coming from the moving parts in the engine. The carbon composition in the deposit shows an increasing tendency as the engine speed and/or load is increased. This actually shows that decarboxylation effect is important during the deposit formation process.