In this paper, we report the effect of nitrogen on the deposition and properties of boron doped diamond films synthesized by hot filament chemical vapor deposition. The diamond films consisting of micro-grains (nano-grains) were realized with low (high) boron source flow rate during the growth processes. The transition of micro-grains to nano-grains is speculated to be strongly (weekly) related with the boron (nitrogen) flow rate. The grain size and Raman spectral feature vary insignificantly as a function of the nitrogen introduction at a certain boron flow rate. The variation of electron field emission characteristics dependent on nitrogen is different between microcrystalline and nanocrystalline boron doped diamond samples, which are related to the combined phase composition, boron doping level and texture structure. There is an optimum nitrogen proportion to improve the field emission properties of the boron-doped films. [5][6][7] affected by doping have been extensively investigated. However, in previous work, the understanding on how the dopants (mainly refer to N and B) modify the growth and properties of diamond films was mostly emphasized on the sole doping by boron or nitrogen. Recently, there are a few reports on the diamond deposition with co-dopants [8,9]. Hartmann et al. [8] reported that in highly boron-doped micro-diamond, large (small) additions of nitrogen would stabilize the diamond structure (induce more graphite formation). The electronic structure of boron-doped nano-diamond films can be markedly modified by nitrogen and there was an optimized gas proportion to achieve improved field emission [9]. However, the corresponding underlying mechanism is still not very clear. Furthermore, growth features related to the co-doping of boron and nitrogen are desirable to be investigated in detail.In this letter, by hot filament CVD (HFCVD), the microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films with co-introducing boron and nitrogen were synthesized by adjusting the gas proportion of nitrogen and boron in the reaction ambient. The effect of nitrogen on the