A head mounted display apparatus and a backlight adjustment method thereof are disclosed. The head mounted display apparatus comprises a display module, a backlight module, an application processor, an eye image capture apparatus, an infrared (IR) light emitting diode (LED) and application specific integrated circuit (ASIC). The backlight module provides backlight to the display module. The eye image capture apparatus captures an eye image. The IR LED provides auxiliary light source to the eye image capture apparatus. The ASIC calculates a current pupil size according to the eye image. The application processor adjusts backlight brightness of the backlight module according to the current pupil size in a measuring mode.一種頭戴式顯示裝置及其背光調整方法。頭戴式顯示裝置包括顯示模組、背光模組、應用處理器、眼球影像擷取裝置、紅外線發光二極體及特殊應用積體電路。背光模組提供背光至顯示模組。眼球影像擷取裝置擷取眼球影像。紅外線發光二極體提供輔助光至眼球影像擷取裝置。特殊應用積體電路根據眼球影像計算目前瞳孔大小。應用處理器於調整模式下，根據目前瞳孔大小調整背光模組之背光亮度。
