Abstract: To improve the information capacity, the modulation signal usually has the Ka-band (≥10 GHz) frequency and dozens of GHz bandwidth. The half-wave voltage of Mach-Zehder modulator ( MZM) will be obviously changed and the modulation efficiency and optical spectrum characteristic will be influenced. The half-wave voltage of the MZM was measured based on the spectrometer method. The spectral characteristic of the MZM was analyzed theoretically and the half-wave voltage of the MZM was calculated through the intensity ratio of modulation sideband. The half-wave voltages were measured experimentally when loading radio frequency ( RF ) signal with different frequencies and powers on the MZM. Results show that the half-wave voltage of the MZM increases with the increase of the modulation frequency. Furthermore, the changing of modulation frequency has large influence on the half-wave voltage than that of the modulation power. Measuring the half-wave voltage of the MZM with the different modulation conditions can support the optimization of the microwave photonic systems, and it is of significance to improve the performance of the systems.