The boron-doped nanocrystalline diamond films were prepared on Si(100) substrates by microwave plasma chemical vapor deposition in gas mixture of CH4/H2/trimethylboron (TMB) with B/C ratio in the range of 0-1900ppm. The dependencies of surface morphology, microstructure, phase composition and field electron emission properties on the B/C ratio were systematically investigated by scanning electron microscope, X-ray diffractometer, visible and UV Raman spectroscopy. The results show that the diamond grains gather together forming ball-like clusters with inhomogeneous size, the doped boron atoms can promote the growth of plane (111) surface and terminate the diamond growth sites, resulting in the reduction of growth rate with the increase of B/C ratio in the gas mixture. The two peaks located at approximately 500 and 1220cm-1 resulted from Fano interference were observed in the visible Raman spectra for the heavily boron-doped nanocrystalline diamond film, and the sp2/sp3 ratio of carbon bonds increased with B/C ratio increasing in gas mixture. The field electron emission performances of the boron-doped nanocrystalline diamond films were obviously dependent on B/C ratio in the gas mixture, and boron doping can improve their field electron emission properties remarkably. The low turn-on electric field of 7.6V/μm was achieved for the boron-doped nanocrystalline diamond film deposited at B/C ratio of 1900ppm.