Abstract：After the detailed analysis of some factors influencing upon the diffusion coefficient of rock，the relation between diffusion coefficient and porosity is deduced from theoretical and observed data. Then a functional relation between the diffusion co- efficient and acoustic interval transit time may be obtained by using relation between porosity and acoustic interval transit time. Authors determined diffusion coefficient of Carboniferous mudstone in mid Tarim basin by taking acoustic interval transit times and above relations. This method allows geophysicist to obtain a great deal of diffusion coefficients of rock and to analyse diffusion coefficient in horizontal direction. Thus it is of great significance to both evaluating the mud ability to stop gas in diffusion phase and estimating the gas diffusion amount.
