The present study numerically investigates the formation mechanism of a tulip-shaped flame observed in a high aspect ratio rectangular channel. The compressible Navier-Stokes equations are solved with a detailed chemical kinetics mechanism of hydrogen. The result indicates that a first trigger for the tulip flame formation is a vortex generation near the left end-wall during the initial stage of a finger-shaped flame development. A large vortex structure produced in the burnt region interacts with the propagating flame front and the vortex-induced acceleration and deceleration of the flame front consequently forms the tulip-shaped flame. The interaction of pressure waves with the flame front affects a flame instability and flame propagation behavior. However, the flame instability may have no significant influence on the formation of tulip-shaped flame in the present condition.