In order to improve the operation reliability of oscillator, a novel radiation-hardened-by-design VCO (voltage-controlled-oscillator) using interleaved structure was presented. The VCO consists of multi-stage delay cells which apply interleaved structure and can complete majority decision voter, so it is tolerant to single-event transient effect. Additionally, the VCO can generate symmetric multi-phase output as no additional module for majority decision voter is needed to be introduced in it. The radiation-harden differential VCO was implemented in a 130 nm bulk complementary metal oxide semiconductor process. Simulation results show that, when deposited charges range from 100 fC to 800 fC, the maximum phase displacement generated by a single simulated ion strike for the proposed VCO output is less than 0.35 rad.