In this study, the morphologies and properties of basalt fibre (BF) treated at different temperatures in air atmosphere were studied. The results showed that the heat treatment caused the formation of crystals on fibre surface, which resulted in the alteration of BF from the paramagnetic material to the ferromagnetic one. The tensile strength of BF was reduced significantly upon thermal treatment, and such reduction occurred in three temperature ranges: i) Decomposition of organic sizing on fibre surface, which led to the exposure of more defect structures (100-400 degrees C), and the structural changes of BF may also occur; ii) Simultaneous relaxation of excessive enthalpy and structural anisotropy, which were responsible for the phase separation in the amorphous matrix in the fibre (500-600 degrees C); iii) Structural modification due to the formation of multifarious crystals in the fibre (700-1000 degrees C). The crystallization in BF began with spontaneous spinel phase formation, which was favored by the initial phase separation and the oxidation of Fe2+. The spinel crystals became nucleation sites for the formation of pyroxene structure, and this process was accompanied by the enrichment of Ca, Mg and Fe on fibre surface. Therefore, the inhibition of crystallization for BF was important to maintain the mechanical performance of filament under the continuous thermal conditions for practical applications.