This study presents a new numerical analysis of thermal behavior and flow of filling gas in inclined double plane windows by considering radiation effects of fluid, as a gray, absorbing, emitting, and scattering medium. In recent years, the installation of inclined double pane windows from the vertical to horizontal sense, especially in the new architecture, is more used. The main goal is to verify the effect of window's inclination angle on the performance of double pane windows in decreasing the rate of heat transfer via this part of the building. The governing equations include the continuity, momentum, and energy, are discretized by using the finite volume method and they are solved with the SIMPLE algorithm. In order to compute the radiative term in the gas energy equation, the radiative transfer equation is solved numerically by the discrete ordinate method. Results are shown as contours of streamlines, isotherms, and distributions of horizontal and vertical components of velocity in the whole cavity of the window and filling gas in different incline angles. The results illustrated that by increasing in incline angle, the rate of flow vortices is decreased. The flow of gas is rotational and the recirculated flow inside the window breaks down to many smaller vortices at a specified inclination angle so it influences the amount of total heat transfer coefficient of the window.