Dynamic Molecular Collision (DMC) model is extended to calculate collisions of N2 and He. An intermolecular potential is obtained as the sum of the two potentials between each atom of a N2 molecule and He. Lennard-Jones (12-6) potential is used as the interatomic potential and potential parameters are determined by combination rule. N2-He collisions are simulated in many cases by Molecular Dynamics (MD) method in order to construct the collision model between N2 and He. A collision cross section is determined based on a diffusion coefficient and a probability density function of energy after collision is determined by the MD method. In the present paper, moreover, this model is applied to simulations of the free jet expansion of N2-He mixture by Direct Simulation Monte Carlo (DSMC) method and the flow field is analyzed. Especially the number density and energy distributions at the axis of a free jet are analyzed in detail.