Abstract: In order to study the mechanism of the rubber effect on drying shrinkage of cement based materials, rubberized mortars were chosen as the study object. Capillary tension theory was used in the study to analysis the factors that influence the drying shrinkage of mortars. The mercury intrusion method was used to study the porosity of rubberized mortars, and the elastic modulus and the drying shrinkage tests were done as well. The results show that rubber especially that of small size can reduce the elastic modulus of cement mortars and increase the porosity and drying shrinkage of mortars. Based on the test results, K_E is defined as the reduction factor of rubber on elastic modulus of mortar and K_h is defined as the increase factor of rubber on micropores (< 50 nm). δ_mr is defined as the rubber effect on mortar drying shrinkage, which is fitting according to K_E and K_h.