Some caculated results had simulated the current field and sea water discharge processes passing two mouths in O Loan dam in rain season. The result shows that, the discharge between dam and sea is passed only by northern mouth when the southern mouth was not opened. Average discharge is about 190 m3/s. The maximum current speed round in 1,7 m/s. In case the southern mouth is opened, the inside and outside dam discharge is mostly passed by the southern mouth. Average discharger passing this mouth is about 135 m3/s. Meanwhile, the average discharge at northern mouth decreases only to 58 m3/s, 4 times decreasing when compering with southern mouth being closed in case.