The evaluation of reinforced concrete slab of a two-direction slab depends on loads, boundary conditions, nature of rebar, concrete and particular the stiffness of its supporting beams. In this article, the author aims to handle with the evaluation of deflection of reinforced concrete slab when its dimensions or the cross- section of supporting beams change. The author recognizes that when the relative stiffness between slab and its supporting beams (Jb/Js) is less than 1.2, it is necessary to consider the effects of these. This conclusion is an important in designing structures because an adeqau appropriate choice ofthat dimension is always complex and is a time-taking work.