This paper deals with a transition between CPW and rectangular waveguide suitable in phased array antenna environment. In case of phased array antenna where the scan angles are very large, the sub array size gets reduced. Hence in such cases the feed element size becomes very small and the accommodation of active module comprising phase shifter and amplifier at the back side of the radiator is not possible. Parametric study for different design parameters of transition and its impact on return loss bandwidth is presented. Wideband transition between CPW and rectangular waveguide using fin-line is achieved.