Thermal properties of materials and surface convection coefficient during freezing or thawing process are the most important factors that influence process rate and qualities of frozen food. The aim of this study is to build mathematical model of thawing process of frozen fish by enthalpy method and use inverse heat transfer problem to estimate its thermal physical properties. In the study, the thawing curve of fish obtained by the explicit and the implicit finite difference methods were accomplished, and by the use of Lavenberg-Marquardt method and Conjugate gradient method the most fitting surface convection coefficient and enthalpy of frozen fish during process could obtain and both methods worked equally well.