Free convection heat transfer from a short vertical plate (with vertical length l and thickness d) was numerically analyzed by the finite difference method. The present results regarding average Nusselt number on a thin vertical plate (d bsime O) are closely approximated by the following equation: [numerical formula] This relation between Nul and Grl shows the same tendency as the experimental results and gives slightly lower Nusselt number. In the range of l and d considered in the present study (d/l≦1 etc.), using length l+d in both Nusselt and Grashof numbers instead of l, average Nusselt numbers on the vertical plate with finite thickness can be approximated by the above correlation equation with an error within about 6%.