A novel etching effect of hydrogen ions on the growth of diamond films was observed. TheH+ion bombardment was performed by applying a negative substrate bias during a microwave plasma chemical vapor deposition process, using only hydrogen as a reactant gas. The effect of this bombardment was investigated by means of scanning electron microscopy. It was found that the etching efficiency ofH+ions on non-[001]-oriented grains is more significant than that on grains with their (001) faces parallel to the substrate. A lateral growth of the (001) faces can occur during the bombardment process. As a result, the size of (001) faces increases afterH+etching while grains with other directions are etched off. This effect provides a way to improve the orientation degree of [001] oriented diamond films and might be helpful for obtaining [001] oriented diamond films with small thickness. ©1997 American Institute of Physics.