A method of determining the type of disk loaded on an optical disk drive, and an optical disk drive using the method are provided. The disk decision method includes operations of detecting a change in the magnitude of a focus error signal generated during a focus search for a recording layer of a disk closest to an outermost layer of the disk, wherein the change in the magnitude of the focus error signal depends on a change in a recording layer of the disk for which aberration correction is performed, and determining a recording layer architecture of the disk according to the detected change in the magnitude of the focus error signal.