This study investigates the pore structure of cement paste after exposing to elevated temperatures, up to 800°C. The variation of pore structure of each sample was monitored for 180 days by mercury intrusion porosimetry (MIP). Test results showed that the increase of heated temperatures causes the increase of the size and volume of pores, which is probably due to the dehydration of hydration products, such as calcium silicate hydrate and calcium hydroxide.