Using a newly developed large ring-shear apparatus, a series of tests was conducted on silty soils to study their undrained shear behaviour, The samples were prepared by mixing loess into a fine-grained silica sand with 10%, 20%, and 30% loess by weight. By performing tests at different initial void ratios, the undrained shear behaviour of samples with different loess contents at loose and medium-dense states was presented and discussed, Based on the tests results, the effects of loess contents on the peak and residual shear strengths are examined. It was found that adding loess into the host sand could lower the peak and residual shear strengths and that these decrease with increasing loess content. Meanwhile, with increasing loess content, the dissipation of generated pore pressure from the shear zone becomes remarkably slower when the shear box was turned into drained conditions, and there is little, if any, dissipation for tests on the mixture with 30% loess.