The effect of limestone powder on microstructure of concrete was studied by using mercury intrusion porosimetry (MIP), backscattering scanning electron (BSE), scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques in this paper.Results show that the compressive strength of concrete containing 100kg/m3 limestone powder can meet the strength requirement.Limestone powder has not pozzolanic activity; it is still unhydrated at the age of 28 days.But its filling effect can make the paste matrix and the interfacial transition zone between matrix and aggregate denser, which will improve the performance of concrete.