Boron-doped polycrystaline diamond films grown by hot-filament-assisted chemical vapor deposition were studied with ultraviolet photoemission spectroscopy (UPS), Raman spectroscopy, X-ray diffractometry and current voltage measurements. The UPS measurement shows that the work function (phi) without electric field is about 3.9 eV . The field-emission current-voltage measurements indicate a threshold voltage ranging from 8.97x106 to 9.64x106 V=m and a work function (phi) about 0.3 eV . These results show that boron doped diamond films exhibit a negative electron affinity in high electric field.