Transitions and pressure drop characteristics of flow in periodically grooved channel are numerically investigated by using finite difference method. The channels consist of fundamental channel elements which have concave or convex rectangular grooves on the smooth plate. The flow fields are assumed to be two-dimensional and to be periodically fully developed. We have examined the effect of combination of the fundamental channel elements on transitions and pressure drop characteristics. The critical Reynolds numbers where a steady state flow bifurcates to a self-sustained oscillatory flow and the corresponding frequencies are evaluated for the combined channels. It is found that the self-sustained oscillatory flow occurs as a result of Hopf bifurcation. As for the pressure drop characteristics, it is found that the pressure drop for the combined channel which consists of elements with concave and convex grooves takes a middle value between the channel of concave elements and that of convex ones.