A computer system and a method for calculating straightness of facial are provided. In the method, an image is processed to obtain a facial area and eyes locations. The facial area is divided into two sub-areas. Then, each of the sub-area and the corresponding symmetric image thereof are coupled together to form two reference images. Finally, the characteristic values of the facial area and two of the reference images are used for calculating the straightness of the facial area based on the principle of facial symmetry. As a result, the judgment on the straightness of the facial in an image can be more correct and objective, and the situation of taking an image with a tilted angle of facial can be avoided.