  A diffusion process associated with the real sub-Laplacian $\Delta_b$, the
real part of the complex Kohn-Spencer Laplacian $\square_b$, on a strictly
pseudoconvex CR manifold has been constructed. In this paper, we investigate
diagonal short time asymptotics of the heat kernel corresponding to the
diffusion process by using Watanabe's asymptotic expansion and give a
representation for the asymptotic expansion of heat kernels which shows a
relationship to the geometric structure.
