Abstract: This paper dealt with the in-situ test results on the deformability of rock masses in nine hydroelectric projects. The shape of the pressure-deformation curves obtained in these tests are discussed and classified in five fundamental types. The deformation characteristics of rock masses are analysed for each type, showing that the lack of the rigidity of the measuring device may lead to considerable error in the test results, and the shape of the pressure-deformation curves may change from one type to another. Finally, the author's suggestion about the improvement of test technique and of the analysis of test results are presented.