Transitional buoyant jet diffusion flames are investigated by 3D spatial direct numerical simulations (DNS). To examine the fuel source and sidewall geometry effects, a comparative study has been performed with four computational cases: a free-square and a free-round case in an open boundary domain, and a corner-square and a corner-round case in a half-confined domain. A grid system of 192 X 192 X 288 nodes is employed to obtain the near field flame structure. Due to the absolute buoyancy instability and the geometry effects, the jet diffusion flames display unsteady dynamic behavior. The free-square case is more vortical and entrains more efficiently than the free-round case because of the self-induced vortex deformation associated with the noncircular source geometry. The sidewall effects are found to be significant on the flame structure because of the wall-induced vortex deformation. Compared with the free cases, the corner cases have a stronger tendency of transition to turbulence. The corner-round case has a much stronger wall-induced vortex deformation than the corner-square case. Although the corner cases are half-confined in the cross-streamwise directions, entrainment of the corner cases is not lower than that of the free cases due to the wall-induced vortex deformation. In contrast with the comparison between the free-round and free-square cases, entrainment of the corner-round case is higher than that of the corner-square case. (C) 2003 The Combustion Institute. All rights reserved.