Two types of in-situ triaxial testing apparatus to measure stress-strain relationship of rock mass (i.e.
large-scale apparatus and small-scale apparatus) are introduced. The large-scale apparatus comprises of
inner and outer cells to apply confining pressure to the specimen, axial loading system which is generally
installed in the ground surface, special instrumentation devices and data recording unit. The small-scale
apparatus consists of a triaxial cell to measure deformations of the specimen under confining pressures and
axial load and a separate axial loading system, which enables to apply axial load to the specimen in any
depths. Axial and radial deformations of the specimen can be measured inside the triaxial cell and
recorded with a data recording unit. For drilling the borehole, preparation of the specimen and performing
the in-situ triaxial test, a special procedure is introduced. Some typical results of proof tests were presented. The results, which were similar to conventional laboratory triaxial tests, showed that the proposed test method was successful to measure average stress-strain relationships of a rock mass.