An oscillatory motion of natural convection in a rectangular cavity has been studied both experimentally and analytically. Experiments were conducted mainly in the case that the hot and cold surfaces were set up as the upper and lower half walls of a side plate, and the other walls were insulated. A periodic oscillation of the hot and cold cells was found to occur by the flow visualization in a certain range of the Rayleigh numbers. The influences of the Prandtl number and the aspect ratios of the cavity on the oscillation frequency were also examined. Unsteady two-dimensional equations, which govern a laminar natural convection, were solved numerically to simulate the oscillation. The calculated flow patterns and oscillation frequencies were in fairly good agreement with the experiments. From these facts, the applicability of above numerical schemes to the other systems of the oscillations was discussed.