Abstract: In the deep geologicalrepositoryfor the disposalof high radioactive nuclearwaste, buffer/back-fill material system works as engineering barrier, hydraulic barrier, chemical barrier and transmissionof heat generated by nuclear waste, which is an efficient guaranteefor the long-term safety and stabilityof the repository. Historicalresults show that bentonite is an ideal buffer/back-fill material. Based on a review of lab tests results of bentonite, the characteristics of water retention and expansive pressure of Gaomiaozi bentonite, which has been chosen as the first choice for using as buffer/back-fill materials in deep disposal of high radioactive waste in China, has been tested and analyzed under unconfined and confined conditions by using osmotic and vapourphase techniques. Results show that, for lower suction, there is a clear difference in water retention properties of densely compacted Gaomiaozi bentonite between confined and unconfined conditions. For the Gaomaiozi Bentonite tested here, there is an exponential relationship between swelling pressure and dry density, and dry density is an important external factor to swelling pressure. All results obtained here are very important to the study of the swelling property of Gaomiaozi bentonite.