In this letter, a novel method of reducing mutual coupling is proposed, and it used on a modified compact broadband antipodal Vivaldi antenna (AVA) array for future 5G millimeter-wave (mmWave) communication application. The proposed structure consists of eight antenna elements that are fed by a 1-to-8 power divider. In order to reduce the mutual coupling between AVA array elements, multiple notch structures are added on the ground plane. Thus, the isolation between the antenna elements can achieve a maximal additional 37.3 dB enhancement, impedance bandwidth is extended slightly from 24.65-28.5 GHz to 24.55-28.5 GHz, and the gain is improved simultaneously. To verify the designed method, the proposed AVA arrays were fabricated and measured. They show an overall size of 28.823 mm × 60 mm × 0.787 mm. The measured gain of the modified AVA array is 6.96-11.32 dB in the frequency band of future 5G mmWave communication, which is higher than the initial AVA array, whose gain is 5.34-8.5 dB.