A nonlinear Lamb wave approach is presented for characterizing the in-plane elastic anisotropy of a solid plate. The effect of second-harmonic generation in an anisotropic solid plate exists and through this the Lamb waves propagate. When the direction of the Lamb wave propagation in an anisotropic plate changes, the influences of the elastic anisotropy of the plate material on the second-harmonic generation of the Lamb wave propagation have been analyzed. Theoretical analyses show that the effect of second-harmonic generation of the Lamb wave propagation is closely associated with the elastic anisotropy of the solid plate. Based on the theoretical analyses, characterization of the in-plane elastic anisotropy of a given rolled aluminum sheet is experimentally studied. For the different directions of the Lamb wave propagation relative to the rolling direction of the aluminum sheet, the amplitude-frequency curves both of the fundamental waves and the second harmonics of Lamb wave propagation are measured under the condition that the Lamb waves have a strong nonlinearity. It is found that the in-plane elastic anisotropy of the rolled aluminum sheet can clearly affect the efficiency of second-harmonic generation by the Lamb wave propagation. The stress wave factors (SWFs) in acousto-ultrasonic technique are used for reference. Based on the data measured, the normalized SWF curves of the Lamb wave propagation versus the orientation angles relative to the rolling direction of the aluminum sheet are obtained. The results show that the second-harmonic SWF of the Lamb wave propagation varies more sensitively with the orientation angles than does the SWF of the fundamental Lamb wave propagation. It is found that the effect of second-harmonic generation of the Lamb wave propagation can be used to accurately characterize the in-plane elastic anisotropy for the given solid plate.