A portable device and a screen brightness control method thereof are provided. The portable device and related method can maintain the current screen brightness in overall consideration of a detected ambient illuminance, a touch of an input unit, and a user's direct touch. The portable device detects an ambient illuminance. If the detected ambient illuminance is less than a threshold value, the portable device calculates a first distance between an illuminance sensor and a touch of an input unit detected from a touch screen, and also calculates a second distance between the illuminance sensor and a user's direct touch detected from the touch screen. If the first distance is equal to or greater than the second distance, the portable device controls the touch screen to maintain the current screen brightness thereof.