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The effect of trace cerium on microstructures and mechanical properties of 690MPa steel plates used for engineering machinery was researched by the comparative analysis of the microstructures, inclusions, and comprehensive mechanical properties of the steel plates after rolled, quenched and tempered. The result shows that trace cerium can improve the yield strength, tensile strength of rolled and quenched steel plates, and low temperature impact toughness of the tempered steel plate significantly, but has little effect on the yield strength and tensile strength of the tempered steel plates. The cerium-bearing steel has finer grain size, less inclusion contents and more spheroidal than that of cerium-free steel. The yield strength of 10 megapascal, the tensile strength of 20 megapascal for rolled plates and the yield strength of 130 megapascal, the tensile strength of 100 megapascal for quenched plates was improved by the addition of cerium. In spite of this, the cerium has little effect on the yield and tensile strength but improved about 50 Joule impact energy values under-40 centigrade degree temperature after the plate was tempered at the temperature of 620 centigrade degree. It has the same rules of the plates tempered at the temperature of 580 centigrade degree.