A study was carried out, using heat flow calorimetry, XRD, FTIR and SEM imaging analysis, of different influences which were exerted by the type of polymer, on the hydration behaviors of cement pastes.It is shown that both SBR and HPMC can cut down the cement hydration heat evolution rate and total heat between 5 h and 20 h.The former has a weaker influence but can play a positive role in enhancing the latter.HPMC also has stronger retardation ability of making the acceleration period ending later for 3.4 h compared with the reference cement.It is amazing that this retardation effect enlarges to 8.2 h for the cement paste modified with a blend of SBR and HPMC.XRD and FTIR analysis show that SBR and HPMC have played a negative role on the contents of Ca(OH)2 in cement pastes cured for 24h, and there is a relationship of mutual promotion between SBR and HPMC in terms of inhibiting the formation and growth of Ca(OH)2, but this inhibiting effect becomes weaker at 72h.Few apparent Ca(OH)2 crystals and some scattered polymer films are found in polymer modified cement pastes from the SEM imaging analysis.So, it can be concluded that a synergistic or accumulative retardation effect exists between SBR and HPMC on cement hydration at early age, and that HPMC has the superior effect owing to its special O-H groups compared to SBR.The existence of the O-H groups enables HPMC to chemically combine with Ca2+, decreasing the Ca2+ concentration in the cement pastes, finally enhancing its retardation effect on cement hydration, rather than relying solely on the physical adsorption effect from SBR to prevent the formation and growth of Ca(OH)2.