The internal tensile stress in polycrystalline diamond films deposited on silicon substrates has been measured from 100 to 700 K by a vibrating‐membrane method. The stress increases strongly with temperature, in a manner consistent with the elastic accommodation of the differential thermal strain between diamond and silicon. The results indicate that the films contain a tensile growth stress of about 500 MPa at the deposition temperature of 1123 K. Derived values of the biaxial elastic modulus fall in the range 730–850 GPa.