An absolute-type encoder includes an absolute-type encoder wheel. The absolute-type encoder wheel includes a primary optical grating unit and two secondary optical grating units, wherein the two secondary optical grating units are arranged outside and inside the primary optical grating unit and contain one less optical grating than that of the primary optical grating unit. The encoder wheel is rotated to find a zero point in the light detection signal of the secondary optical grating unit, and a corresponding light detection value of the primary optical grating unit is also determined based on the zero point. The absolute position of the encoder wheel can be calculated with reference to the corresponding light detection value. The absolute-type encoder can also be used as an incremental-type encoder to provide more versatility.