Modifying effects of carboxyl terminated styrene butadiene emulsion D623 on physical properties of cement mortar, such as water retention and air void content of fresh mortar and porosity, pore size distribution and fracture morphology of hardened mortar, were studied. D623 improves water retention of fresh mortar and entrains a large amount of micro air void which causes bulk density to reduce, pore size distribution to change. It is obvious that a network structure of polymer films has formed and crisscrossed with cement hydrates. Porosity of hardened mortar has no relation with air void content, but a little relation with bulk density of fresh mortar. Compressive strength, flexural strength and elastic modulus have good relations with porosity and bulk density. Tensile bond strength, however,is not same as the former.