Objective In order to study the intrinsic mechanism of doping borane during the growth of nano-crystalline diamond films to achieve precise control of the process. Methods Nano-crystalline diamond ( NCD) films were boron doped by the microwave plasma enhanced chemical vapor deposition ( MPCVD) method using hydrogen diluted di-borane as boron source. The influences of borane concentration on grain size, surface roughness, surface resistance and boron atom concentration of boron-doped NCD films were investigated. Results The surface roughness and grain size of nanocrystalline diamond films increased with increasing concentrations of boron source. The surface resistance first showed a downward trend with increasing boron concentrations, and then gradually reached equilibrium. Conclusion The results showed that doping of boron could improve the surface conductivity performance of nano-crystalline diamond films and increase its surface roughness and grain size. By comprehensive comparison, the optimal borane concentration was 0. 02% under the doping condition of 700 ℃ and 15 min.