In this paper, we developed a hybrid elastic wave method to determine the anisotropic constants of a thin fiber-reinforced composite plate. The method is based on the measurements of the pure mode bulk ultrasonic wave velocities as well as the Lamb wave velocities. The formulae of the pure mode plane wave propagation in an orthotropic medium are summarized, first. Then, the numerical results of Lamb wave dispersion in a thin fiber-reinforced plate are presented. In the experimental part, conventional ultrasonic velocity measurements were conducted to obtain the pure mode velocities of a unidirectional fiber-reinforced composite and therefore the three corresponding elastic constants. The remaining two elastic constants of the thin composite plate were then obtained by using the inverse analyses of the Lamb wave dispersion. We note that with this hybrid method, some of the anisotropic constants can be measured accurately with well-developed bulk wave ultrasonics, while the rest of the undetermined elastic constants can be obtained from the inversion of the Lamb wave dispersion.