The Forward-Difference Method was applied to the one-dimensional diffusion equation for a problem with constant surface and initial solute concentrations and diffusivity independent of concentration. Two interesting observations were made on the error between the exact analytic solution and the numerical solution : (1) when x equals root 2Dt, the Forward-Difference Method becomes very accurate no matter what value of lambda (= D Delta t/Delta x(2)) was used; (2) the maximum error, which occurs at similar to 2.76 root Dt, increased linearly with lambda.