The objective of this research was to investigate the effect of water stress in regulated deficit irrigation (RDI) on the yield of soybean growing on Ultisol soil. This research was conducted under plastic house on the experimental farm of Lampung Polytechnique from August to November 2004. The water stress treatments in regulated deficit irrigation were ET1 (1.0 × ETc), ET2 (0.8 × ETc), ET3 (0.6 × ETc), ET4 (0.4 × ETc) and ET5 (0.2 × ETc), arranged in a randomized block design with four replications. ETc means crop evapotranspiration under standard condition, which was well watered. For example, the ET2 (0.8 × ETc) treatment means that the amount of supplied water per a day is the same as the crop adjustment evapotranspiration (ETcadj) with the value 0.8 of water stress coefficient (K
s). The RDI treatments were carried out just at vegetative phase and its treatments were stopped at the beginning of flowering phase, and afterwards the treatments were watered at 1.0 × ETc. The results showed that since week II, the soybean experienced stress throughout the growth period except ET2 treatment. ET2 treatment started to be stressed at week V and continued to be stressed until the harvest time. At the ET3 treatment, the critical water content (θc) of soybean was reached at week II, and the θc was 0.24 m3/m3 on the average. The RDI at vegetative period significantly affected the yield. The highest yield was ET1 (35.2 g/plant), followed by ET2 (31.0 g/plant), ET3 (18.1 g/plant), ET4 (7.6 g/plant), and ET5 (3.3 g/plant). The optimal water management of soybean with the highest yield efficiency was regulated deficit irrigation with water stress coefficient (K
s) of 0.80 for vegetative phase.