Recently, several studies focus on a GaN material system that exhibits a significant probability of use in power devices including wide-gap semiconductors. However, the GaN-HEMT is also a structure that easily leads to crystal defects in AlGaN and i-GaN heterojunction. The aim of the study involved investigating the cause of the current collapse in GaN-HEMT after device construction. A GaN-HEMT with a field plate structure was subject to an environmental temperature change from 300 K to 400 K. A pulse voltage was applied to the gate electrode, and the transient response characteristic of the drain current was analyzed. Given the application of the pulse voltage on the gate electrode, charging and discharging of 2 DEG carriers was repeated with respect to crystal defects near the gate electrode. The charge / discharge reduction was observed via a sampling oscilloscope as a transient response. The transient response exhibited an evident dependence on temperature change. The dependence indicated a time constant change, and thus it was possible to calculate the activation energy of crystal defects trapping carriers. The results suggested that the crystal defect evaluation of GaN-HEMT was possible via transient response analysis of 2DEG carrier by using the proposed method.