A new approach for determining the biaxial modulus of chemical vapor-deposited diamond films is presented, with an example of diamond coating on copper substrate. The biaxial modulus of the film and substrate are calculated from a combination of bimetal and plate bending theories. To realize a valid calculation, residual stresses in the film and the curvature radius of the sample need to be known independently. Raman spectroscopy is employed to evaluate residual stress and a laser displacement meter is used to measure the sample deflection. Raman spectra of the as-grown diamond show that the film is under a compressive stress of -5.1 GPa, while the corresponding freestanding film obtained by etching off the metal substrate is nearly stress-free. The sample surface is almost flat before the diamond coating, while the curvature radius becomes 124 mm after diamond deposition. The biaxial modulus calculated for the diamond film is approximately 995 GPa. This implies that the diamond film is of relatively high quality, which is confirmed by its Raman spectra. Interestingly, the biaxial modulus calculated for the copper substrate is much lower than the value given in the literature. This is attributed to plastic deformation of the substrate. A concept of equivalent modulus is discussed.