Based on a diesel engine with common rail system, influence of EGR on combustion and emission characteristic were investigated. The test results show that EGR can cut down both cylinder pressure and the rate of pressure rise, which leads to the depression of instantaneous rate of heat release and combustion temperature and the extension of the whole combustion period.EGR make effective improvements on reducing NOx emissions in all the operating modes. In light load condition CO and HC emissions take a favorable turn with larger EGR rate, SOOT and fuel consumption are well maintained. In moderate and heavy load condition, the curve of HC ,CO emissions assume “hook-like”, which is get down first and then steep rise. SOOT is well maintained before the inflection point but get deteriorate after the point. As the working load rate of the engine become heavier, the rate of EGR whose inflection point located becomes smaller and the fuel consumption turn out to be worse.