Abstract: In order to improve the utilization of blast furnace slag and steel slag into high strength building materials to great extent, the effect of slag particle size on the compressive strength of cementitious systems prepared with a great deal of blast furnace and steel slag was investigated by grey correlation analysis. The blast furnace slag and steel slag account for 50% and 30% in mass respectively with the water-binder ratio of 0.34. Slag particles of less than 8.39 μm in size can contribute to the 3 d and 28 d compressive strengths, but the particles of more than 8.39 μm in size have negative effect on them. It is indicated that slag particles with the size between 5.03 μm and 8.39 μm should be increased in the cementitious system so as to improve the 28 d compressive strength.