Transient laminar natural convection in a two-dimensional enclosure partitioned by an adiabatic baffle is investigated numerically, A penalty finite-element method with a Newton-Raphson iteration algorithm and a backward difference scheme dealing with the time term are adopted to solve governing equations. The effects of the baffle and Rayleigh number on the heat transfer mechanism are found to be substantial during the transient process for Rayleigh numbers of 104and 106. However, the variations of the heat transfer mechanism occur mainly in the first one-third of the time period of the transient, in spite of the presence, absence, or location of a baffle.