Fracture strength of CVD diamond films deposited by the hot-filament chemical vapor deposition method on sintered SiC substrate was estimated using the indentation method and FEM analysis. The authors determined the critical indentation force to cause ring crack in the diamond film correctly utilizing both of the acoustic emission (AE) and corrosion potential fluctuation (CPF) method during indentation of Rockwell indenter of natural single crystal diamond with radius of 400 p.m. The polarity distribution of the first arrival symmetric mode of the Lamb wave AEs was utilized for the classification of the Mode-I ring crack and the Mode-II interfacial exfoliation. The CPF measurement in the acidified solution was possible due to weak electrical conductivity of CVD diamond and SiC. The critical indentation force for the micro-meter size polycrystalline diamond (MCD) and nano-meter size diamond (NCD) films with different thicknesses were accurately determined by AE and/or CPF. Fracture strength of these films were estimated by FEM using the critical indentation forces. The fracture strength of the MCD was estimated as 6.4 GPa for 21 p.m thick MCD and 4.0 GPa for 70 pm thick MCD film. Fracture strengths of these films were supposed to be determined by the transngranular cleavage crack, while that (6.1 GPa) of the NCD film was determined by weak graphite phase along grain boundaries.