This study deals with the derivation of the diffusion equation by considering the displacement of a fluid particle through an unsaturated porous medium as a Markov's stochastic process.The comparison of this equation with that obtained from the combination of Darcy's generalized equation and the equation of continuity allows us to find the physical significance of the diffusivity coefficient D as well as its partial derivative (ð D)/(ðx).The physical interpretation of these parameters may be related to the boundary conditions of the flow phenomenon and enables us to determine the field of application and the validity limits of the diffusion theory.