Solid-state fermentation of sweet sorghum stalk is a cost-effective technology for bioethanol production. Sugar transfer from the interior of sweet sorghum plant cell to the surface is one of the key factors affecting the fermentation process because the system takes place in the absence of free water. Mass transfer of sugar from the interior of the sweet sorghum to the surface of substrates is mainly driven by the sugar concentration gradient. A mass transfer model based on three steps process was developed. Taking into consideration of the different tissue structures of sweet sorghum stalks, a two simultaneous first-order kinetic model of sugar transfer with two parameters characterizing the tissue structure difference was developed to describe the sugar transfer process in sweet sorghum. Solid-liquid extraction experiments were used to gather experimental kinetics data of sugar transfer. Four factors impacting the mass transfer of sugar including particle sizes, stirring speed, temperature, and osmotic pressure were investigated. The results showed that the newly modified model fitted well the kinetics of sugar transfer in different varieties of sweet sorghum. This model could be helpful in optimizing solid state fermentation of sweet sorghum stalks. Moreover, the kinetic model with the added structural parameters based on difference in plant tissue structure can also be used to describe the extraction of any active substance from types of feedstocks with similar chemical compositions and biomass structures. (c) 2015 Elsevier Ltd. All rights reserved.