It is very important to evaluate gas transfer velocity and the surface velocity divergence in the open-channel flows such as artificial flume and natural river. It is however hard to obtain universal form of gas transfer velocity in rough-bottom open-channel flows by using original surface divergence model. The present study focused on the interfacial gas transfer in the open-channel with strip roughness and found that the large surface velocity divergence is induced by shedding vortices around roughness element. Particularly, the gas transfer depends significantly on relative spacing of strip roughness. Furthermore, we examined whether a modified surface divergence model predicts reasonably the gas transfer velocity, irrespective of bottom roughness conditions in open-channel flows.