Summary: Natural convection of air in a three-dimensional rectangular cavity heated from below is numerically simulated using SIMPLE algorithm with a QUICK scheme. The different results occur when different initial conditions are given. But the influence of initial conditions appears only in some range. The influence of lateral boundaries becomes slighter when convection becomes stronger. According to the comparison of three-dimensional and two-dimensional results, the flow patterns and heat transfer in the central part of the cavity can be assumed as two-dimensional flow.