In the deposition chamber of a double tubed coaxial line type microwave plasma chemical vapor deposition system, plasma parameters are measured with a cylindrical probe by varying the area of the substrate table to which DC bias voltage is applied. Electron temperature and electron density remain constant when the area of the substrate table and DC bias voltage are varied. It has been clarified that the dependence of plasma space potential on DC bias voltage varies with varying the area of the substrate table only within a range of the positive DC bias voltage. The saturation current density on the cylindrical probe differs from those on substrate tables. In addition, the saturation current densities on substrate tables differ from each other. It has been clarified that these differences in the saturation current densities can be explained by considering the thickness of single layer sheath formed on the cylindrical probe and the substrate tables.