Chemical vapor deposited (CVD) diamond films exhibiting stable and intrinsic negative electron affinity (NEA) surfaces are expected to have practical uses in field emission display devices. In this work, we attempt to study how NEA diamond surfaces can be removed and re-established by various surface treatments. The emission current density can be enhanced from 0.8 to 100 muA/cm(2) at an electric field of 18 V/mum by hydrogenation treatment. The field emission current of diamond surfaces varies manifestly with temperature as studied in this experiment which is contradictory to the simple Fowler-Nordheim equation. The field emission equation for CVD diamond surfaces at low field and high temperatures derived in this work agrees satisfactorily with the experimental results.