Optical constants effect on conjugate laminar natural convection and radiative heat transfer in an air filled rectangular enclosure with one vertical isothermal wall and one vertical semitransparent wall was numerically investigated. The steady-state continuity, Navier-Stokes and energy equations were carried out by the finite volume method, and the Discrete Ordinates Method (DOM) was used to solve the radiative heat transfer equation. Semitransparent radiation effect of vertical wall on laminar natural convection in rectangular enclosure was compared with non-semitransparent for Ra = 10(7) and Pr = 0.71. Detailed results including flow profiles, temperature fields, and Nusselt numbers were presented. The influence of optical constants of semitransparent wall on natural convection and radiation heat transfer in enclosure were also solved, considering different refractive indexes and absorption coefficients of semitransparent wall. It is found that optical constants effect of semitransparent wall on laminar natural convection in rectangular enclosure is marked. (C) 2013 Elsevier Ltd. All rights reserved.