Abstract: An on-line thickness gauge (OLTG) and method are described herein that are capable of measuring a thickness of a moving glass substrate. In the preferred embodiment, the OLTG includes a Y-guide and a stabilizing unit that respectively captures and stabilizes the moving glass substrate. The OLTG also includes a laser instrument which contains a laser source and a detector. The laser source emits a beam at the front surface of the moving glass substrate. And, the detector receives two beams one of which was reflected by the front surface of the moving glass substrate and the other beam which was reflected by the back surface of the moving glass substrate. The OLTG further includes a processor that analyzes the two beams received by the detector to determine a distance between the two beams which is then used to determine the thickness of the moving glass substrate.