Smith-Purcell (SP) radiations of weakly relativistic oversized backward wave oscillator (BWO) are examined experimentally. Our oversized BWOs are driven by weakly relativistic electron beams less than 100 keV. New element of this work is an enhancement of SP radiation in the oversized BWO using the bunching section. An annular beam is self-bunched in the K-band bunching section and then is injected into the oversized K-band BWO. SP radiations in the BWO are strongly enhanced in the U-band, which is about twice of the bunching frequency. Moreover, the oversized BWO is developed to G-band. By using the G-band bunching, the SP radiations are enhanced in the H-band. The enhancement of SP-radiation in the oversized BWO by the self-bunched electron beam is attractive to develop THz wave sources.