1. A device for determining a gesture, comprising: a light emitting unit configured to emit a light beam; an image sensing device configured to capture an image of a hand reflecting the light beam; and a processing circuit, electrically connected to the image sensing device, configured to obtain the image and determine a gesture of the hand by performing an operation on the image; wherein the operation comprises steps of: selecting pixels in the image having a brightness greater than or equal to a brightness threshold; sorting the selected pixels; selecting a first predetermined percentage of pixels from the sorted pixels; dividing the adjacent pixels in the first predetermined percentage of pixels into a same group; and determining the gesture of the hand according to the number of groups of pixels.