In this study, the improvement mechanisms of fluidity and strength development of the steam cured polymer-modified mortar (PMM) with low water cement ratio were investigated. The fluidity was estimated from the rheological property, the flocculation formation structure of the fresh PMM and the rate of hydration reaction. The strength development was estimated from the hydration reaction of cement, the hydration products, and the composition and structure of hardened PMM. The results obtained are as follows : The rheological property of PMM for high self-compactablility was optimized by adjusting of polymer and superplaticizer contents. The transition zone was not observed and the uniformly dense hydration structure was formed in PMM because of its low water to cement ratio. The polymer in PMM filled pore space by mesh structure. PMM with high early strength Portland cement accelerates the hydration reaction and forms more dense structure compare to that with ordinary Portland cement.