A number of previous experimental studies showed that polymer geogrid alone as well as sand alone exhibit significantly viscous behaviour. It is shown that the viscous response of geogrid-reinforced sand is also significant due to the viscous properties of both geogrid and sand. Due to their extremely complicated interaction, the investigation of this issue only by experimental study is insufficient. A non-linear three-component model that is known to be able to simulate very well the viscous behaviour of soil was implemented into a non-linear elasto-plastic FE code. The viscous behaviour of geogrid observed in tensile tests and that of sand and geogrid-reinforced sand observed in plane strain compression tests were successfully simulated by the FEM analysis. The FEM analysis showed that the tensile force of geogrid during sustained loading of reinforced sand decreases with time.