Development of a numerical model which describes launching of offshore jackets from barge is presented in this paper. In this model, in addition to capabilities of commercial softwares, water entry forces on jacket members and an implicit Newmark solution technique are included. The results are in general agreement with other numerical softwares available (SACS). Fluid forces acting on jacket and the importance of each one is discussed. It is observed that water entry forces on horizontal jacket members are very significant and may locally govern the design of these members. This force is more important for horizontal slender members near the mud-line, which do not experience significant environmental loading in operating conditions. Therefore the water entry impact force with large magnitude can cause over-stress and/or ovalling of near mud-line members. It is also observed that taking water entry forces in account modifies the jacket trajectory only in a little extent.