A numerical study of three-dimensional flow and heat transfer on a one period of a rectangular plain with built-in delta winglet type longitudinal vortex generators is carried out. Longitudinal vortices develop along the side edge of the delta winglets due to the pressure difference between the front surface (facing the flow) and back surface. These vortices interact with thermal boundary layer and produce a three dimensional swirling flow that mixes near wall fluid with the midstream. Thus the thermal boundary layer is disrupted and heat transfer is enhanced. The efficiency of the delta winglet vortex generators widely varies depending on their size and shape, as well as the locations where they are implemented. In the present study, the longitudinal vortices have been created by the delta-winglet type vortex generators in common-flow-down configuration.The investigation are carried out for four different angles of attack of the winglets (15°, 25°, 35° and 45°) and different Reynolds number (1000 to 4000) for common flow down configuration. It is observed from the simulations result among different angles of attack of the VGs, the performance of delta winglet pair at 25° is best in terms of heat transfer at the same Reynolds number.