The influence of oxygen deficiency on the superconductivity and the thermoelectric power in single phase YBa2Cu3O7-x system was studied. The results show that the superconducting transition temperature decreases, and the magnitude of the thermoelectric power increases with the amount of oxygen vacancies. From the experimental results we suggest that holes conduction is predominant in single phase YBa2Cu3O7-x system, the relatively large phonon-drag thermopower indicates that there exists strong electron-phonon interaction in YBa2Cu3O7-x superconductors.