Based on process calibration, single event transient in VoltageControlled Oscillators of high frequency PhaseLocked Loops in a 0.18μm bulk process is studied by the TCAD Mixedmode simulation. The impact of voltage, LET and temperature on SET are studied. Our simulation results demonstrate that the max phase difference caused by the particles on VCO is weak when NMOS works in the cutoff region. The pulse error number caused by the particles on VCO increases linearly as LET increases, and when the device temperature increases, the max phase difference on VCO also generally increases.