We describe a simple microwave‐produced plasma source that has been constructed for plasma‐assisted chemical vapor deposition of diamond thin films. Microwave power from a 700 W domestic microwave oven magnetron is fed into a water‐cooled cylindrical stainless steel vacuum vessel. A methane/hydrogen gas mixture introduced into the vessel is excited by the microwaves to produce a well‐defined plasma ball which does not interact with the walls of the vessel. The position of the plasma ball in the vessel can be predicted by cold cavity calculations of the axial electric field profile. The vessel has several diagnostic ports which do not alter its resonant condition. Diamond has been deposited on various substrates, placed beneath the plasma ball on a graphite‐capped quartz pedestal. Some of these results are presented and discussed.