The purpose of this study is to analyze the natural convection heat transfer between horizontal parallel plates in a thermoforming process. Natural convection in a narrow cavity space formed by two horizontal plates is very different from that of an almost infinite space. For an efficient application of natural convection to cooling processes, it is necessary to fully understand the mechanisms of heat dissipation inside parallel-plate channels. Basic knowledge on the flows and heat transfer of natural convection in such configurations is needed for the thermal design of free convection oven. In the present work, numerical simulations were conducted with an input condition for a heat transfer computer code with laminar flow in a rectangular cavity with free atmospheric wall where the initial temperature of ceramic parts and atmospheric air were 373.15 K and 300 K, respectively. The Rayleigh numbers is fixed at R_a = 4 × 10⁴ without considering solar radiation. The air inside the cavity was assumed radiatively non-participant.