The microstructures and phase composition of as-cast AZ91+xLa alloys (x=0%, 0.3%, 0.5%, 0.7%, 1.0%, 1.5% and 2.0%, mass fraction) were investigated by SEM and XRD, and their mechanical properties were tested at room temperature. When 0.3%-2.0% La was added into the AZ91 alloys, Al11La3 phase was precipitated along the grain boundaries and interdendritic boundaries, and its morphology changed from acicular to flake gradually as the La content was increased. Meanwhile the size and volume fraction of β-Mg17Al12 phase was decreased. Moreover, the grain sizes of α-Mg were greatly reduced with La addition, and the best refinement is obtained in alloys containing 1.0%-1.5% La. Mechanical properties of AZ91 magnesium alloy can be improved significantly by addition of La, which results from the fine-grain strengthening of La, the optimization of the morphology and amount of β-Mg17Al12 phase and dispersion strengthening etc. Among all the investigated alloys, the AZ91+1.5%La alloy exhibits the optimum mechanical properties, i.e. the ultimate tensile strength is 226 MPa and the elongation 7.5% at room temperature.