Varied water diffusivities estimated by Crank's equation have been reported. However, the corresponding theoretical explanation is still not available. Here reported is our effort about this concern. A model combining the Fick's second law diffusion with a convective boundary condition on the material surface is solved numerically, and the obtained solution is used to estimate water diffusivity through Crank equation. The result shows that a significant underestimation of water diffusivity occurs due to a failure to account for appropriate boundary, which may imply that Crank's equation need be further improved. (C) 2012 Elsevier B.V. All rights reserved.