Apparatus and method for displaying screen in portable terminal

An apparatus and a method for displaying a screen image of a mobile terminal are provided to enhance the visibility of an output screen image by varying color quality according to external brightness. A display unit(104) outputs a screen image such as status information, limited numbers and characters, videos, still images generated during an operation of a mobile terminal. An illumination sensing unit(102) measures external brightness. A visibility determining unit(106) checks visibility color quality, namely, pre-set color quality corresponding to the measured external brightness. When a visibility adjustment event occurs, a controller(100) checks the visibility color quality corresponding to the measured external brightness via the visibility determining unit(106) and controls to change currently set color quality to the visibility color quality so as to be outputted to the display unit(104).