An empirical formula, which demonstrates the interference fringe visibility as a function of the slit separation and the temperature of thermal atoms, is obtained by analysing a recent experiment [B1och et al. Nature 403 (2000)166] that refers to the spatial correlation function of a trapped Bose gas below the critical temperature. We find that the decay rate of the coherence function for the non-condensed component is about two orders larger than that of the pure condensed component. The changes of the interference fringe visibility versus the falling time for different temperatures and slit separations are also discussed.