Applying the radiation-hardened-by-design (RHBD) technique to improve the bias generator and the ring oscillator of the differential voltage-controlled oscillator (VCO) with symmetrical loads, a single-event transient (SET) hardened VCO was designed and implemented in 0.18 μm CMOS process based on the failure mechanisms. Simulation results indicate that the single-event susceptibility of the VCO is significantly reduced. Simultaneously, it also reduces the jitter sensitivity to supply noise. This new VCO topology results in a decrease in the frequency, but it can be figured out by adjusting the sizes of the delay buffer. Furthermore, the radiation hardened VCO leads to a decreased area requirement.