The thermal decomposition behaviour of ammonium pentaborate octahydrate (APB) and its conversion to boron oxide were investigated using isothermal thermogravimetry, non-isothermal thermogravimetry and differential thermal analysis. TG and DTA curves of APB were obtained in its own decomposition atmosphere. According to these curves, the thermal decomposition of APB to boron oxide occurs in two main steps : the first step corresponds to the dehydration and the second is attributed to the deamination. It was found that the shapes of the TG curves recorded under isothermal conditions at different temperatures are strongly affected by the working temperature. Also, two endothermic peaks were observed on the DTA curve of APB, representing dehydration and deamination processes, respectively.