A new method is proposed to measure the toughness of thin films prepared on substrates. By partly removing the substrate from the edge of film and substrate, the overhang part of the film projected out of the edge of substrate is prepared as the specimen to measure the film toughness. External mechanical load is applied on this specimen by pushing down both sides of the notch, which is grooved from the free edge of the film to be the starter of crack extension. The crack extension behavior is recorded as the load-crack extension diagram, which is compared to the results of numerical simulation for the the crack extension with the assumed value of toughness. With the correspondence of experiment to simulation, the optimum value of assumed toughness can be determined as the result of measurement. With this method, the toughness of a diamond film deposited on WC-Co was obtained.