An analytical model for the behaviour of masonry under in-plane loading is presented. The masonry units and the mortar joints are modelled separately each with its own non-linear material characteristics. The main advantage of the model is the successful consideration of the transverse principal stress which is developed during the in-plane loading of masonry. For this purpose a triaxial material model has been developed and is presented briefly. The results from the analysis of masonry models are compared with the experimental findings from the literature and a very good agreement is found. Finally a parametric analysis of uniaxially loaded masonry panels until failure is carried out. The resulting curves of masonry strength versus mortar strength and joint thickness are presented and commented.