A theoretical model of a laminar diffusion flame at the leading edge of a fuel plate in a forced convective flow is presented and solved numerically to study the flame stabilization and blowoff phenomena. The system of governing equations consists of the two-dimensional Navier-Stokes momentum, energy and species equations with a one-step overall chemical reaction and second-order, finite rate Arrhenius kinetics. The computation is performed over a wide range of Damkohler numbers. For large Damkohler numbers, envelope flames are found to exist where the computed fuel evaporation rate, the flame stand-off distance and the velocity profiles show certain similitude. As the Damkohler number is lowered, a transition to open-tip flame takes place where the flame becomes stabilized on the sides of the fuel plate. Further decreasing of the Damkohler number pushes the diffusion flame downstream out of the leading edge region. In this paper, the flame structures of the envelope and the open-tip flames are presented together with a description of the transition sequence. The implication of this work to downstream boundary layer combustion is also discussed.