The vibration of a left vertical hot wall in a square cavity with thermally insulated vertical walls facing unsteady natural convection is investigated numerically. The cavity is filled with an ideal gas and the top wall is exposed to free stream conditions. Using the primitive variables of velocity and pressure, the staggered grid technique and the marker-and-cell (MAC) method is used to solve the governing equations using the Boussinesq approximation for natural convection. The numerical solution is obtained by using Matlab platform. Sample results are shown in the form of contour plots for pressure, velocity vectors, vorticity, and temperature fields for fixed values of Reynolds number. Detailed analyses of unsteady laminar flow and thermal fields are exhibited over broad ranges of Reynolds number and frequency of the oscillating wall. Systematically-organized computational results based on the MAC method with an explicit formulation indicate enhancement of heat transfer demonstrated by higher average Nusselt number values for selected values of the Reynolds number.