The microstructures of Portland cement paste (water to cement ratio is 0.4, curing time is from 1 day to 28 days) are simulated based on the numerical cement hydration model, HUMOSTRUC3D (van Breugel, 1991;Koenders,1997;Ye, 2003).The nanostructures of inner and outer C-S-H are simulated by the packing of mono-sized (5 nm) spheres.The pore structures (capillary pores and gel pores) of Portland cement paste are established by upgrading the simulated nanostructures of C-S-H to the simulated microstructures of Portland cement paste.The pore size distribution of Portland cement paste is simulated by using the image segmentation method (Shapiro and Stockman, 2001) to analyse the simulated pore structures of Portland cement paste.The simulation results indicate that the pore size distribution of the simulated capillary pores of Portland cement paste at the age of 1 day to 28 days is in a good agreement with the pore size distribution determined by scanning electron microscopy (SEM).The pore size distribution of the simulated gel pores of Portland cement paste (interlayer gel pores of outer C-S-H and gel pores of inner C-S-H are not included) is validated by the pore size distribution obtained by mercury intrusion porosimetry (MIP).The pores with pore size of 20 nm to 100 nm occupy very small volume fraction in the simulated Portland cement paste at each curing time (0.69% to 1.38%).This is consistent with the experimental results obtained by nuclear magnetic resonance (NMR).