Microwave plasma-enhanced chemical-vapor-deposited (MPECVD) diamond films grown under various bias conditions applied to the substrates have been examined by electron microscopy.. It is noticed that in a certain range, increasing the bias voltage can effectively reduce the diamond grain size. Transmission electron microscopy (TEM) images distinctly reveal that the diamond films consisting of ultrafine crystallites of about 7-10 nm can be successfully fabricated by applying an appropriate bias voltage. Comparing the conventional diamond films deposited without, bias: and the nanocrystalline diamond films grown with a bias, one can see that the latter have better field emission properties.