A numerical investigation of laminar natural convection heat transfer and flow in two-dimensional wavy rectangular enclosures is presented. The enclosure is heated from the flat bottom wall and cooled from the wavy (undulated) top wall which is expressed by sinusoidal functions. The lateral walls are adiabatic. The numerical simulations were carried out by the commercially available CFD software-namely the FLUENT (R). The steady-state continuity, Navier-Stokes and energy equations which are subjected to the Boussinesq approximation are solved using the SIMPLE algorithm. The convergence criteria for all equations are set to 10(-5). The Rayleigh numbers which are considered in the numerical simulations ranged from 5x10(4) to 10(7) while the value 0.7 was used for the Prandtl number of air. The enclosures with aspect ratios of L/H=1, 2, 4 and 8 were studied. The period, lambda, for the sinusoidal functions imposed on the wavy wall was 1.5, 2.5 and 4.5 while the wave-amplitude was kept constant at a/H=0.1. The temperature field and the flow structure were analyzed for varying Rayleigh numbers. The aspect ratio, L/H, the number of undulations of the wavy wall, and the mean Nusselt numbers obtained from the bottom wall are computed and analyzed for each case.