The growth and field emission properties of nanodiamond films have been investigated. Uniform nanodiamond films with an average grain size of 10-60 nm have been successfully prepared by microwave plasma-enhanced chemical vapor deposition technique using a gas mixture of nitrogen-methane-hydrogen. Field emission results show that the threshold electric field increases, whereas the emission current density and the number of emission sites decrease with increasing mean diamond grain size. The film with a small grain size of 10 nm has a threshold electric field of 1.5 V/mum, and reaches an emission current of 780 muA/cm(2) at electric field of 3.5 V/mum, demonstrating that small grain size nanodiamond films are promising material for low-field electron emitters.,