The mercury intrusion porosimetry (MIP) was used to investigate the pore structure characteristics of fly-ash cement pastes with two water to binder ratios (w/b=0.3, 0.5). The total porosity, pore fraction distribution, contact angle hysteresis and the pore entrapment are quantified from the intrusion/extrusion data. The tortuosity of pore structure is further calculated from pore entrapment. The results show: (1) the w/b ratio is determinant for the formation of pore structure patterns, high w/b ratio providing high porosity, high connectivity but low pore surface roughness to pore structure; (2) the contact angle hysteresis can be related to pore surface roughness, the hysteresis factor α_θ attaining respectively 0.42, 0.75 for Paste I (w/b=0.5) and Paste II (w/b=0.3) samples at 180d; (3) the tortuosity and the porosity obey just roughly the power law and more pore characteristics are needed to describe the tortuosity of blended cement pastes.