To fulfill the need of measurement on the small bore′s interior features, a specially designed optical transmission component was adopted to divert exterior illumination into the interior of small bores and to export the interior image. This study surpassed the traditional mode and achieved the exterior placement of the measurement sensors to make it possible for the measurement in narrow space. To fulfill the need of measurement on small bores in different position and direction, a new flexible in-line measurement system was set up utilizing the movement platform of industrial robots. Firstly, the design of the measurement system was proposed based on analysis of preliminary method, and the flow of the work was given. Then, the question of alignment between sensor and sample was analyzed in detail, and a novel, reasonable method of aligning was given. Based on the system, the measuring experiment was carried out on 0.6 mm, 1.0 mm and 2.0 mm simulated defect points on interior wall of 10 mm diameter bore, the standard deviation is all less than 0.01 mm. The experimental result shows that the system is valid and able to accomplish the feasible, on-line measurement.