Dielectric, P-E hysteresis loop, and X-ray diffraction measurements of the (Pb1-xSrx)(Mg1/2W1/2)O-3 ceramics system have been conducted in order to study the effect of A-site atomic substitution on phase transition. It is found that an antiferroelectric intermediate phase begins to appear with the addition of Sr. The temperature range of the intermediate phase increases with the Sr concentration. The transition temperature of the intermediate phase region from the paraelectric state is shown to be nearly independent of Sr content. However, the transition temperature into a low temperature antiferroelectric state from an intermediate phase region is found to be strongly dependent on the concentration of Sr. The transition behavior of the system is related to two modulations based on X-ray diffraction analyses.