A new method is proposed to measure the toughness of brittle thin films prepared on substrates. By partly etching the substrate under one edge of film, an overhang part of the film projecting out over the edge of substrate was prepared as the specimen to measure the film toughness. Mechanical load was applied on this specimen by pushing down both sides of the notch, which was grooved from the free edge of the film to be the starter of crack extension. The crack extension behavior was recorded as the load-crack extension diagram, which was compared to the results of numerical simulations for the crack extension with the assumed value of the toughness. With the correspondence of the load-crack extension diagrams which were obtained by both experiment and simulation, the optimum value of assumed toughness could be determined as the result of measurement. As an example of practical application, the toughness of diamond films deposited on silicon substrate was obtained.