Hydration of cement in the presence of SBR dispersion and powder respectively was investigated using the methods of ITC, XRD and ESEM. The results show that both the dispersion and powder of SBR facilitate the formation, enhance the stability of AFt and inhibit the formation of C4AH13 in cement paste; the effect of the powder is more evident than the dispersion. Both the dispersion and powder of SBR delay the formation of C-S-H and Ca(OH)2 in cement paste, and the effect of the dispersion is more evident. Up to 3 days, the structure of the SBR dispersion – or powder – modified cement pastes has no significant difference with that of control paste except due to a thin polymer film on the surface. The two polymers delay the early cement hydration, but have no significant effect after 3 days.