1. A gesture detecting method adapted to an optical touch device, the optical touch device comprising an indication plane and two image sensing units disposed at two corners of one side of the indication plane, the gesture detecting method comprising: sensing two images of a gesture by the two image sensing units, wherein the gesture is performed on the indication plane; determining whether at least two touch points exist in one of the two images; if at least two touch points exist in one of the two images, generating a quadrangle according to a far left boundary and a far right boundary of each of the two images; calculating a reference radius of a reference circle corresponding to the quadrangle; and determining whether the gesture is a grab gesture according to a variance of the reference radius.