Abstract: The measuring principle and procedure of coalescent force between thin solid films and substrates by scratching method were briefly introduced. Some important factors influencing the final experimental result, the accuracy of measured critical load (Lc) were discussed. By classifying the film-substrate systems and summarizing a great deal of experimental data, it was concluded that the physical and chemical characteristics of films and substrates are very important parameters for the scratching experiments. For some special film-substrate systems, great errors of measured critical loads might be obtained when using sound emission signal only, and the reasons were analyzed and discuessed. It was suggested that reasonably using the sound emission signal, slope of friction force curve and microscope observation to determinate the position of critical load are very necessary.