1. A method of detecting subject of image, comprising the follow steps of: providing an image capturing module to capture a temporal image; providing an image processing module to receive the temporal image and decide a plurality of objects from the temporal image; using the image processing module to define a plurality of regions on the temporal image based on a center of the temporal image; using the image processing module to assign an initial setup value to each of the objects according to the region corresponding to each of the objects; using the image processing module to execute an operation to each of the objects to increase or decrease each of the initial setup values; using the image processing module to execute a sorting based on each of the initial setup values which have already modified by the operation; and selecting at least one of the objects as a subject of the temporal image based on each of the initial setup values after the sorting.