Abstract: The mechanical properties and the fractions of individual phases at microscale were measured by nanoindentation and back scattered electron (BSE) image analysis, and they can be used as the input in modeling the mechanical properties of cement paste. The mechanical properties of cement paste were measured by microindentation. The effect of individual phases on the effective modulus of hardened cement paste was quantified by comparing the measured and homogenized paste modulus. It is found that the effective modulus of pore needs to be set to 12 GPa as input for homogenization. Meanwhile, the high density hydrated calcium silicate gel (HD-CSH) and the low density hydrated calcium silicate gel (LD-CSH) need to be viewed as two different phases in modeling. Whereas, different clinkers can be regarded a single phase. The effective paste moduli calculated by both Mori-Tanaka scheme and self-consistent scheme agree well with the measured paste modulus.