The paper deals with the modelling of the undrained response of non-cohesive partially saturated soils subjected to triaxial compression. The model proposed is based on an incremental equation describing the pre-failure response of non-cohesive soils during shearing. The original model, developed by Sawicki, was modified by taking into account pore fluid compressibility. The governing equation makes it possible to simulate effective stress paths under undrained conditions. Numerical results are compared with experimental data.