In this paper, the effect of ground granulated blast-furnace slag on the pore structure and compressive strength of hardened cement pastes are investigated. Five different fineness of ground granulated blast-furnace slag whose Blaine's specific surface areas are about 4000, 8000, 11000, 18000 and 30000 crnVg respectively, are used as a partial replacement for normal portland cement by 15, 30, 50 and 70%. As the ground granulated blast-furnace slag with high Blaine's specific surface areas has high filling role and high rate of hydration at early ages, the porosity, threshold radius and the early strength of hardened cement pastes increases with the increase of fineness of ground granulated blast-furnace slag. Furthermore, the effects of particle size of ground granulated blast-furnace slag, combined water, porosity and threshold radius on compressive strength of hardened cement pastes containing ground granulated blast-furnace slag are examined.