The dynamic analysis of a semi-submersible was investigated both numerically and experimentally in regular and irregular waves. In the numerical model, the equilibrium conditions were developed and solved in time domain using an iterative incremental numerical scheme. The wave force calculations were based on the Morison Equation applied at the instantaneous position of the structure. Mooring system contribution to dynamic behavior was evaluated numerically using nonlinear catenary equations. Experimentally, a twin hulled semi-submersible model in a scale of 1100 was tested in regular and irregular waves with the model oriented in the head, beam and quartering directions. The experimental measurements validated the accuracy of the numerical model. © 2010 IEEE.