A simplified constitutive model called a Swinging Plane Model is presented for monotonic and cyclic soil response including liquefaction. This model is based on two mobilized planes: a plane of maximum shear stress, which swings, and a horizontal plane which is spatially fixed. By controlling two mobilized planes, the model can simulate the principal stress rotation effect associated with simple shear from different K₀ states, which can significantly influence soil behaviour. The proposed model gives a similar skeleton behaviour for soils having the same mean stress, regardless of K₀ conditions as observed in laboratory tests. The soil skeleton behaviour observed in cyclic drained simple shear tests, including compaction during unloading and dilation at large strain is captured in the model. Undrained monotonic and cyclic response is predicted by imposing the volumetric constraint of the water on the drained or skeleton behaviour. This constitutive model is incorporated into the dynamic coupled stress-flow finite difference program FLAC (Fast Lagrangian Analysis of Continua). The model was first calibrated with drained monotonic and cyclic simple shear tests on Fraser River sand, and verified by comparing predicted and measured undrained monotonic and cyclic behaviour of Fraser River sand.