The effects of MoO3 additives on the clinkers mineral formation of portland cement were investigated by X-ray diffraction,scanning electron microscopy. One reference mixture and others modified mixtures, prepared by mixing the blank sample with chemical grade MoO3 were studied. Based on experimental result, MoO3 shows the less positive effect, MoO3 added less than 1wt% was benefit for f-CaO absorbtion, but when more than 1wt% MoO3 was added, with the temperature increasing, increasing the free CaO (f-CaO) content compared with the pure sample,which cound not promote the formation of 3CaO·SiO4. MoO3 can stabilizes crystalline structure of β-2CaO·SiO4. The analysis results for the microstructure indicate that grain size of C3S in the samples with MoO3 is larger than that of only -portland clinker.