The mechanical strength of chemically vapor deposited diamond is characterized in terms of fracture toughness. The toughness of film and adhesion interface to the substrate were independently measured using techniques recently developed by the authors. In this paper, diamond was deposited by microwave plasma chemical vapor deposition using a variety of deposition conditions. The fracture toughness of the interface initially increased and then decreased with increasing amounts of methane in the source gas mixture. On the contrary, the toughness of the films was simply found to decrease with respect to the increment of methane concentration. This interesting trend of toughness was explained by the crystalline structure of the deposited diamond.