The undrained shear strength of partially saturated sand is examined based on a series of undrained triaxial compression and extension tests on Toyoura sand. The effects of partial saturation, density, and triaxial compression/extension modes are discussed in detail. The conditions of “flow” and “no flow” are categorised in terms of contractive and dilative behaviours. The threshold values of the undrained shear strength ratio dividing “flow” and “no flow” are found to be independent of the B-value and triaxial compression/extension modes. The empirical relations of the B-value against the initial state ratio rc and the undrained shear strength ratio Sus/p’c are discussed in detail.