Abstract: The mechanical property of super-high pressure gasket material-pyrophyllite is studied in this paper. Based on the analysis of the press shear and the disc-squeezing methods, a new material test technique-pure internal slipping method is suggested. The principal advantage of this method is that the friction between the sample and dies has no influence on test results, so that the test results are perfectly determined by the plastic property of samples. According to this idea, a simple device is set up and a series of tests on the mechanical property of pyrophylite are carried out. On the other hand, a program for stress distribution analysis in the plastic region of samples by slip-line theory is worked out. By using this program, the relation between the average pressure p, cohesion K internal friction angle and the diameter-thickness ratio d/h can be calculated, and the procedure for calculating the material parameter K, from the test results p-d/h is suggested. The test results show that the fired temperature and the moisture have strong influence on the mechanical property of samples and some rules exist between them.