This paper is aimed to present the design and feasibility investigations of adopting the available on-site optical inspection system, which is commonly used for steel plate dimension measurement, to supply on-line dynamic gap measurements of a non-contacting conveyance structure in a steel mill. Adequate software and hardware implementations based on digital image processing techniques have been adapted to the entire system formulations and estimations. Results show that the system can supply accurate and rapid gap measurements and thus can fulfill the design and operational objectives.