A new discrete vortex model is developed and applied to reveal the essential feature of the vortex pairing, a key process to turbulent mixing layer evolution. The model accounts for the finite size of each eddy as a circulation monopole plus a vortex quadrupole and describes a self-rotating elliptic vortex deformable in a stretching flow field in the presence of other vortices. It is demonstrated that the coalescence of pairing vortices takes place, through interaction between a pair of quadrupoles, when the magnitude of the stretching rate in the eddies exceeds that of the self-rotating angular velocity. The streakline pattern outside the eddies, calculated for a temporally developing mixing layer, is in fairly good agreement with existing numerical calcuation results, and is thus expected to provide a powerful analytical tool for simulating turbulent mixing layers.