The responses of voltage-controlled oscillators (VCOs) to single-event transients (SETs) were investigated. A classic differential VCO with symmetrical loads was implemented in a 180nm bulk CMOS process. Modeling the charge deposition and collection from ion strikes as a current source, the transient SET responses of this VCO were achieved by circuit-level simulation. Simulations and analysis indicated SET responses were not only dependent on incident energy and the frequency of oscillation, but also dependent on striking time, which can increase the phase displacement by up to 300 degrees. Additionally, some nodes in the bias generation circuit are most sensitive to SET. Strikes in these nodes were amplified and resulted in an approximate 7 clock cycles distortion.