A tungsten-needle substrate was perpendicularly set onto a substrate holder. Diamond films were grown on the substrate by varying the length of the needle inserted into plasma. The results demonstrated that when the bias voltage was -50 V and the methane concentration was 5%, a diamond thin film grew for a needle length of 0.5 mm, and granular diamond grew for a needle length of 1 mm. Plasma potentials were measured while the bias voltage was varied. The plasma potential became negative at bias voltage of 0 V, and it changed to positive as the negative bias voltage increased. These results revealed that diamonds were grown near the boundary between the transition region surrounding the plasma ball and the ion sheath. In addition, the results also indicated that the bias voltage can control the density and energy of ions, electrons and radicals near the plasma, and therefore plays a role in providing optimal conditions for diamond growth. (C) 1999 Elsevier Science S.A. All rights reserved.