Polybutadiene-graft-polystyrene (PB-g-PS) latex were prepared via emulsion graft copolymerization method. The weight ratios of polybutadiene to styrene ranged from 70/30, 50/50 to 30/70, then the PB-g-PS latex was used to modify cement mortar. This study examines the effects of core-shell ratios（c/s） on the typical properties and morphology of PB-g-PS latex modified mortars. The PB-g-PS latex modified cement mortar were prepared at different polymer-cement ratios（p/c）, and tested to obtain the water-cement ratios（w/c）, fluidity, water retention rate, flexural and compressive strengths. SEM was used to investigate the microstructure of polymer- modified mortars（PMMs）. On the basis of the test results, effect of the c/s and p/c on the typical properties and morphology of PMMs were discussed. It is concluded that PB-g-PS core-shell latex can be incorporated into cement as modifier.