A method for judging directionality of a housing having two parallel panels by a gravity sensor is disclosed. The method is used for a handheld apparatus to judge which of the panels is upward. The method first obtains a set of component quantity of gravity on three coordinate axes from a three dimensional gravity sensor. When −0.5 g>gz≧−1 g and 1 g≧gz>0.5 g, a first direction status and a second direction status is acquired, respectively. When 0.5 g≧gz≧−0.5 g, the method further reposes on two parameters, polarity of gz and magnitude of the component quantity on y-axis (gy), to judge. When gz>0 and −0.5 g>gy≧−1 g or when gz 0.5 g, the judging result is the first direction status. When gz gy≧1 g or when gz>0 and 1 g≧gy>0.5 g, it is the second one. The rest will be judged to be “don't care”.