Abstract: The polybutadiene-graft-polystyrene (PB-g-PS) latex with a core-shell ratio 50/50 was used to modify cement mortar. The effects of water-cement ratio (W/C) and the polymer-cement ratio (P/C) on the flowability, compressive/flexural strength and water absorption rate of the cement mortar modified with PB-g-PS core-shell structure latex were investigated. Experimental results showed that the flowability of modified mortar increases with increasing P/C and W/C, and the PB-g-PS latex can remarkably reduce the capillary water absorption rate. Although the compressive strength of the PMM (polymer modified mortar) decreases, the flexural strength of part of PMMs increases to a certain extent at low W/C. The microstructural analysis showed that the mortar modified in such way is of more compact structure, thus benefitting the improvement of its performance.