On the Estimation of wind loads on offshore structures which have the deck supported by substructures, it is clear through previous studies that lift force of deck is caused by asymmetry of upper and lower surfaces of deck and that it may be much larger than drag force in case of large waves. Therefore, wind-wave tank experiments on such structures under progressive wave conditions were conducted and the effect of progressive wave on lift force was investigated. Lift forces were measured as a function of substructure, unit number, and wave height. The Experimental results showed that the lift force is significant, especially on leading edge of deck. The study also showed that the fluctuating lift force can be comparable in magnitude to the mean lift force in case of large waves. Further estimation method is provided to help define the nature of lift forces on such structures.