A method is proposed to measure the thickness of nano-scale thin film. Based on the differences about mechanical property between the film and the base, a required scratching will be obtained with the proper tool scratching the film directly, which is through the film touching the base and without any effects to the surface of the base. Scanning the scratching area by using atomic force microscope, some data will be obtained and the average thickness of nano-scale thin film can be calculated in the scratching area. The thickness of the TiO2 nano-scale thin film was measured by this method, the experiment result showed that the average thickness of the film is 71.6 nm, which is consistent with the reported result of the reference. This method has high measuring precision, wider application area, more intuitive images, simpler calculation and operation.