The objective of this study is to evaluate the liquefaction behaviour of fibre reinforced sand concerning the lower and upper limits of medium relative densities of 50% and 70%, respectively. A systematic testing program of the stress-controlled cyclic triaxial tests was performed on saturated sand samples with and without fibre reinforcements under undrained conditions. The confining pressure for all test cases was 100 kPa reflecting the actual overburden pressure in field conditions. The effect of parameters such as fibre content (0.25%, 0.50%, and 1%) and fibre length (6 mm and 12 mm) on the liquefaction behaviour of unreinforced and reinforced specimens was studied. Upon increasing the fibre content and fibre length, the number of loading cycles leading to liquefaction increased. Pore water pressure and shear strain curves are obtained for the fibre reinforced medium dense sands. Furthermore, the boundaries presented in the literature on the sands are shown in comparison with the results of fibre reinforced sands of this study. As a conclusion, the outcomes of this study are useful to develop insight into the behaviour of clean and fibre reinforced medium dense sands under seismic loading conditions. Based on the test results, it was found that the number of loading cycles has significant influence on the generation of excess pore pressure. (C) 2016 The Authors. Published by Elsevier Ltd.