The application of adaptive backstepping technique to the dc-bus voltage regulation and unity-power-factor control for a pulsewidth modulation three-phase ac/dc boost converter is investigated. The controller design is based on a new model derived from power balancing between ac and dc sides. To compensate the nonlinearity in model and counteract uncertainty in load, the adaptive controller that achieves both voltage regulation and unity power factor is systematically derived by using feedback linearization and adaptive backstepping technique. The proposed approach provides direct regulation of dc-link voltage under large perturbation in load. Simulation results provide clear evidence on the voltage reference tracking performance under large perturbation in load.