A rheological model of metal alloy in the semi-solid state used as a raw material in the thixoforming process has been developed. The theoretical side of this casting method and the mathematical model of the material in this state verified by available experimental data were described. This model enables prediction of main phenomena that are encountered during the process of casting and forming of semisolid metal alloys. This includes in particular description of the phase segregation during the flow, thixotropic rheological phenomena with the yield stress, viscosity dependence on the shear stress, solid phase fraction and its agglomeration state. The comparison of the modelling results with experimental data proves that the model sufficiently well describes the thixoforming process.