In this study a planar large-area microwave plasma source is used to grow diamond films at low gas pressure. This plasma source is based on the excitation of plasma surface waves so that overdense plasmas can be generated. Above all, this plasma source is easy to scale up. For admixture of CH4/H-2 gas, radical information and characteristics of the plasma are carefully characterized at low pressure. Some features different from those at high pressure are observed. A three-step process for diamond growth in the planar microwave plasma chemical vapor deposition system has been developed. High nucleation density can be achieved as a result. At a low pressure of 0.2 Torr, diamond films can be successfully deposited on a 4-in. Si(100) wafer, exhibiting a large amount of non-sp(3) bonding. The effects of plasma properties on the diamond film are addressed.