LLC resonant converters have been widely used in industry because of its soft switching characteristics in the full load range. However, due to the increasing switching frequency with the help of wideband-gap devices, electromagnetic interference (EMI) of LLC resonant converters has become a key problem in such systems. High dv/dt or di/dt of wideband-gap (WBG) semiconductor switches produce a large number of high-frequency harmonic components, thus common-mode (CM) and differential-mode (DM) noise signals are generated in LLC resonant converters. In order to investigate EMI issues, an improved CM and DM models are established respectively to represent the high-frequency behavior, and the equivalent circuit parameters of CM and DM impedances are extracted according to the experimental results. The calculated CM and DM impedances are in good agreement with the measured ones from the vector network analyzer (VNA), which verifies the effectiveness of the proposed high-frequency CM and DM models.