This study tried to reduce NOx and particulate emissions simutaneously in a high speed direct-injection diesel engine. Fuel-rich combustion was used to reduce NOx emission at initial combustion stage and high turbulence combustion was used to reduce particulate emission at diffusion combustion stage. Experiments were carried out to investigate the effects of high squish combustion chambers with small throat on combustion process and exhaust emissions. Results showed that in the high squish combustion chamber, both NOx and particulate emissions reduced with retarded injection timing. Endoscopic high speed photography was employed to analyze experimental results. In the original combustion chamber at the early combustion stage, the high luminosity flame tended to spread out of the cavity. Fuel-rich mixture burned slowly over the piston crown. In the high squish combustion chamber, flame rotated violently around the bowl. Fuel-rich mixture was kept beneath the squish lip, and was mainly oxidized inside the bowl. This combustion phenomenon was considered to be one of the reasons for low particulate emission in high squish combustion chambers.