Abstract: The paper presents the particle simulation of peer-to-peer locking for mutually coupled relativistic backward-wave oscillators(BWOs). In the simulation, a pair of relativistic BWOs with identical physical structures are driven by different accelerating voltages, which will work at different states with different oscillatory frequencies and output powers. The results indicate that, the relativistic BWOs connected in parallel with a special coupling slot will lock phase mutually. Their oscillatory frequencies are locked to a common frequency, and their energy conversion efficiencies are improved with an increase in the total output power of no less than 10%. Both their output powers become more stable.