Kinetic studies of char gasification with CO2 were conducted using thermogravimetric analysis (TGA) under isothermal conditions. A set of improved solutions were proposed to amend the data acquisition and processing in an isothermal TGA mode to reduce the processing error. On this basis, the effects of the external and internal mass transfer limitations on the gasification reactivity and kinetic parameters were investigated. Conventional cylindrical alumina crucibles (CAC) with different sidewall heights and a flat alumina disc (FAD) were used for sample loading. The stagnant gas region in the CAC played an important role in increasing the external mass transfer resistance of CO2 from the bulk flow to the bed surface. The external mass transfer limitation was eliminated in the FAD and, therefore, an abundant provision of CO2 to the surface of char bed was achieved. It was found that the char reactivity decreased linearly with increasing the height of the stagnant gas region but decreased exponentially with increasing bed depth. The mass-independent reactivity can be obtained by carefully striking a balance between the bed thickness and the loading area. The external mass transfers that occurred in the stagnant gas region definitely decreased the reactivity, the activation energy and the frequency factor but had a limited influence on the reaction order. The inter-particle diffusion (bed diffusion) had more significant effects on the reactivity and all the kinetic parameters than the external mass transfers.