With the development of high-speed semiconductor devices, the cutoff frequency of high electron mobility transistor (HEMT) can achieve sub-terahertz level, which cannot be simply measured by traditional electrical method. Therefore, the ultrafast optical method is employed to measure the cutoff frequency of HEMT. The femtosecond laser pulse, which has extremely short duration, is used to suddenly turn off the HEMT that is under saturated condition. Furthermore, the terahertz time-domain spectroscopy technology is applied to measure the change of operation current, which can persist even less than 1 ps after the HEMT is switched off. Finally, the cutoff frequency of HEMT can be directly calculated by using the relationship between the terahertz traces (i.e., the curve of the source-drain current versus time) and the cutoff frequency of HEMT which can achieve sub-terahertz level.