Abstract: In this paper, radiations generated by prebunching in FELs are investigated. It is shown that not only the efficiency can be improved, but also the low gain can be enhanced by prebunching, Therefore, the function of a prebuncher in FELs driven by rf linac is explained theoretically. It is also indicated that prebunching can short the laser wavelength without increasing the electron beam energy and relieve the requirement on the electron beam quality.