The kinetics of the thermal degradation of Polyarylene ether natriles (PEN) (crosslinked and uncrosslinked) were investigated by thermogravimetric analysis (TGA). The corresponding kinetic parameters of PEN were determined using Flynn–Wall–Ozawa method and Friedman method, respectively. Satava method was also used to discuss the probable degradation mechanisms of PEN. The results showed that the activation energy obtained from Flynn–Wall–Ozawa method was in good agreement with the value obtained from Friedman method. The solid-state decomposition mechanisms of PEN and crosslinked PEN were A2 type (nucleation and growth) and R2 type (phase boundary controlled reaction), respectively. The activation energy and initial decomposition temperature of crosslinked PEN were higher than that of PEN, which indicates that crosslinking treatment is effective to enhance the thermal stability of PEN.