Colliding interaction of a vortex pair with a premixed flame is numerically studied with a two-step reaction model including chain-branching and chain-breaking reactions. The vortex pair has a maximum circumferential velocity ranging from 4.7 to 54.7 times the laminar burning velocity and has a core diameter ranging from 1.1 to 1.55 times the flame thickness. Besides well-known interacting behaviors such as the flame wrinkling by a weak vortex pair and the pocket formation by a moderate vortex pair, an elongation of the concave flame without entraining the burned gas appears in the interaction with a strong vortex pair. The different behaviors of interaction are attributed to the ratio of the moving velocity of the vortex pair to the burning velocity and the criterion is represented by the ratio of the maximum circumferential velocity to the burning velocity. For moderate and strong vortices, vorticity generation due to the baroclinic effect and reduction of distance between vortex cores result in an acceleration of the vortex pair. Flame propagation reduces the vorticity owing to dilatation of the burned gas and increase of the kinematic viscosity, but it accelerates the moving speed of the vortex pair by decreasing the distance between the vortex cores and increasing the induced velocity. The local stretch rate is dominated by the local strain rate for moderate and strong vortices except at the trailing edge of the mushroom head where the large positive curvature has an effect. The temporal evolution of the curvature, the strain rate and the stretch rate at the stagnation point does not depend on the Lewis number but depends on the vortex strength. However, the local burning velocity at the stagnation point becomes smaller for larger Lewis numbers and stronger vortices. For Le=0.6. there is a delay of response for the positive stretch that is known as the effect of unsteadiness. The increase of global burning rate is generally proportional to the increase of flame surface for Le = 1.0. On the other hand, the global burning rate is amplified by the increase of reaction rate as well as the increase of the flame surface for Le=0.6. and it is reduced by the decrease of reaction rate in spite of the increase of the flame surface for Le=1.6.