The objective of the present investigation was to understand the relationship between the pore structure of gypsum mold and suctionability as a function of the water/plaster ratio in the hydrate reaction of plaster. Nine kinds of gypsum mold, which have different water ratios of hydration, were used.The relationship between the pore characteristics and suctionability of molds were analysed considering the structural difference of the micropores in gypsum molds.It was found that the pore volume and mean pore size increase as the water volume in hydration increased, however, pore size distribution gave a different pattern. That is, volume fraction of the micropore under 1μm in total pore volume decreased as the water/plaster ratio increased, and the bimodal distribution changed into a unimodal shape.Therefore, the suction pressure gave the maximum value for the gypsum sample which was obtained what water was added to 80, 90g with 100g plaster at the reaction of hydration.