A statistical mechanical model for the heterogeneous nucleation process has been constructed in order to give a more solid foundation to the classical model based on continuum thermodynamics. A size correction of the surface tension has been included in the present model, which leads to the evaluation of a microscopic contact angle. The heterogeneous nucleation of water on different substrates has been examined. With the introduction of the concepts of negative line tension and surface diffusion into the statistical mechanical model, a smaller value for the critical supersaturation for heterogeneous nucleation is expected, in agreement with the experimental results, as compared with the predictions of the classical theory. Moreover, our model gives higher nucleation rates for small aerosol particles as compared with the predictions of the classical model of heterogeneous nucleation.