Abstract: To realize phase-locking of high power microwave (HPM) sources for the purposes of spatial power combining, a Ku-band coaxial transit-time oscillator (TTO) with an injection structure is investigated. Between the input cavity and the trinal-cavity buncher, a reflector with reflection efficiency of 99.9% is introduced to prevent the leakage power propagating to the input cavity and the cathode. The RF signal is injected into the input cavity through a coaxial waveguide along the guiding magnet, which avoids separating the magnet coils, and suppresses the asymmetric modes excited in the device. Particle-in-cell (PIC) simulation results indicate that when the injection power ratio ρ is 0.1, the phase-locking is achieved for a frequency difference of 67MHz. It has also been found that the injected signal of higher power results in higher output power and earlier saturation. Moreover, a wider locking bandwidth is expected by the results than that obtained by the Adler’s condition.