Abstract: BCN thin film was deposited on high speed steel substrate by arc enhanced reactive magnetron sputtering (AEMS). Carbon content of the BCN thin film was adjusted by changing graphite target's power. Thermal stability of the BCN thin film was studied by techniques of Fourier-transform infrared absorption, high-resolution transmission electron microscopy, micro-hardness indentation and scratch testing. The results indicate that the BCN thin film had a good thermal stability at elevated temperature. With the increase of carbon content, the atom arrangement order degree of the BCN thin film increased after vacuum annealing at elevated temperature, some of the binding bonds were transformed or decomposed, adhesion strength decreased, and thermal stability of the BCN thin film became poor.