Effects of content of styrene-butadiene copolymer emulsion(SD622S) on the bulk density and mechanical properties of cement mortar with varying curing conditions at fixed water-cement ratio of 0.40 were investigated.The relationship between mechanical properties and bulk density of the modified cement mortar was also studied.The test results show that when addition of SD622S ranges from 0% to 20%, the bulk density reaches minimum at 8% SD622S.With the addition of SD622S lower than 8%,the compressive strength and the flexural strength decrease with increasing SD622S content.With use level below 10% of SD622S,the dynamic elastic modulus decreases with increasing SD622S content.When use level of SD622S is less than 10%,there exits a very close correlation between compressive strength(or dynamic elastic modulus) and bulk density,but the correlation between flexural strength and bulk density is not clear.