This paper presents an experimental study on the effects of Exhaust Gas Recirculation (EGR) on soot emission and diesel knock by using a direct-injection diesel engine. The effect of EGR on soot emission depends on operating conditions. Under the condition that the mass fraction burned by pre-mixed combustion of the two combustion patterns is increased, soot emission is decreased by EGR. It is also revealed that the EGR has the effects of both increasing and decreasing the diesel knock intensity. The mechanisms of these effects of EGR on soot emission and diesel knock are discussed.