Flowslide is a common type of failure of granular soil slopes. Although many flowslides can be explained using static liquefaction or instability behaviour of sand under undrained conditions, some of the failures might have occurred under essentially drained conditions, e.g. the Wachusett Dam case in 1907. Recent laboratory studies on Changi sand under axisymmetric conditions have shown that sand can become unstable under completely drained conditions. Based on these laboratory studies, new failure mechanisms for granular slopes were proposed. However, to date, most of the experiments on the instability behaviour of granular soils were carried out under axisymmetric conditions even though slope failures can only be simplified into plane-strain conditions. Although the behaviour of sand under plane-strain conditions has been studied by a number of researchers, the instability of sand under plane-strain conditions has seldom been investigated. In this paper, experimental data obtained from plane-strain tests are presented to illustrate the unstable behaviour of sand under both undrained and drained conditions. A comparison between the instability conditions observed under axisymmetric and plane-strain conditions is made. A unified instability condition which is applicable to both axisymmetric and plane-strain conditions is presented.