The water content in the specimen volume is controlled by suction in water retentively test. Water retentively test is the load suction by the loading different paths pore water pressure and pore air pressure, and measuring the amount of water absorption at the suction, to investigate the soil water retention. Methods of suction control is considered two methods. This paper reports the investigation was the effect of soil water characteristic curves of different methods of controlling the suction in decomposed granite soil. As a Results of the decomposed granite soil water retentively test by the pressure plate method, there is a difference between controlled only by pore air pressure and controlled by the difference between pore water pressure and pore air pressure in the low suction area.