A 24-30 GHz monolithic integrated Single-Pole Double-Throw (SPDT) switch based on 100 nm Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) process is presented in this paper.This switch adopts a quarter-wavelength microstrip line with parallel HEMT devices topology.By using two-stage parallel HEMT design, the SPDT achieves greater isolation while maintaining good Insertion Loss (IL).With a control voltage of 0/-5 V, the measured IL of this SPDT is less than 1.5 dB, the isolation is greater than 28 dB, and the input power 1 dB compression point is greater than 27 dBm within 24-30 GHz 5G millimeter-wave frequency bands.The measurement results well verify the simulations.