A numerical study has been conducted to investigate the natural convection inside the channel between the flat-plate cover and the sine-wave absorber in a cross-corrugated solar air heater. The effects of major system parameters on the natural convection heat transfer are simulated. These parameters include Delta T, the difference of average temperatures between the cover and the absorber, A, the characteristic height ratio of the channel which is defined as the ratio of the channel height to the amplitude height of the wavelike absorber, L, the characteristic geometric ratio which is defined as the ratio of one-fourth of the wavelength to the amplitude height of the absorber, and theta, the angle of inclination of the heater. The numerical results show that to suppress the natural convection heat loss effectively, A should be larger than 2, L larger than 1 and theta less than 40 degrees.