Disclosed is a method for determining environmental brightness to appropriately control display brightness according to variations of the environmental brightness in different places in a mobile communication terminal including a camera having an automatic gain control (AGC) function, and a method for controlling the display brightness using the same, including the steps of calculating a luminance average of a center region of a photographed screen corresponding to an AGC reference region and a luminance average of an edge region except for the center region as brightness values from an image signal generated by photographing of the camera, comparing the brightness value of the edge region with a previously-set dark environmental critical value, comparing the brightness value of the center region with the brightness value of the edge region according to the comparison result, and determining that the environmental brightness is one of the bright state and the dark state, extracting luminance elements from an image signal which will be transmitted to a display, calculating a histogram of the luminance elements, and performing brightness contrast stretching, controlling the display brightness by adding or subtracting a previously-set control value to/from the brightness contrast stretched pixels according to the determined environmental brightness, and replacing the brightness-controlled luminance elements by the luminance elements of the image signal which will be transmitted to the display.