Chitin is one of the most abuncant biopolymers. There are three polymorphic crystal structures of chitin, that is, α-, β-, γ- structure. In this study, α, β, γ-chitin were isolated from natural resources by treatment with dilute NaOH solution for deprotenization, followed by treatment with dilute HCl solution for demineralization, and characterised by solid state CP/MAS ¹³C NMR and XRD. The thermal degradation properties of chitin has been studied by TGA in the range of 20 ~ 700 ℃. From the isothermal TGA experiments, the initial activation energy and the activation energy at different stage is obtained. The differences in the thermal degradation kinetics of chitins were due to their different intermolecular hydrogen bonding systems.