A device for determining a gesture includes a light emitting unit, an image sensing device and a processing circuit. The light emitting unit emits a light beam. The image sensing device captures an image of a hand reflecting the light beam. The processing circuit obtains the image and determine a gesture of the hand by performing an operation on the image; wherein the operation includes: 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. A method for determining a gesture and an operation method of the aforementioned device are also provided.