In this paper, a new true triaxial loading device with mixed rigid and flexible boundaries and its setup are described. The main features of the new true triaxial loading device are four sliding rigid plates acting on the soil specimen enclosed in a rubber membrane, which solves the corner contact interference problem. Measures for reducing friction on the specimen are also presented. All forces and sources of friction on the soil specimen applied by this loading device and their influence on the distribution of strains and stresses in the specimen are analysed and discussed. The true triaxial apparatus with this new loading device has been used to study the stress-strain behaviour of a completely decomposed granite (CDG) soil in Hong Kong. Typical test results are presented and discussed. Based on the description, test results, and discussion, it is found that this new true loading device has a number of merits, especially for testing soils under large strains without corner interferences. It is also found that the stress-strain behaviour of the CDG is dependent on the b-value and the Lade-Duncan failure criterion is more suitable for the CDG in a threedimensional stress state. Copyright © 2011 by ASTM International.