In this paper a steady-state natural convection was numerically simulated in horizontal rectangular cavity with a sinusoidal bottom wavy wall. This new type of surface rarely studied and investigated. The vertical walls are insulated while the bottom surface maintained to higher temperature than the top surface. In this numerical simulation that based on our elaborated code. the investigated parameters are Rayleigh number that is fixed to Ra=106, the wave length λ varied from 1 to 6 and the amplitude of the wave A is changing from 0.10 to 0.20 with a step of 0.05 were chosen in order to analyze the effect of these parameters on the heat transfer and the flow fields in two dimensional (2-D) enclosure filled with air. It is found that the geometrical parameters has a significant influence on the heat transfer inside the cavity, and also the importance of this study occur in the different applications where this kind of phenomena is present such as cooling, drying systems and solar captors.