A feasibility study to develop a fiber-optic probe for measuring features of an internal thread has been carried out. The principle used in this study is the optical triangulation mapping technique. A design of the gage including the optical system is discussed. The design of the optical components is evaluated by use of a computer ray-tracing pogram. In addition, signal detection error analysis is performed to estimate the accuracy of the measurement. Results indicate that the gage accuracy is well within ±2.5 um, which is the desired goal that is meaningful for applications in quality control, quality assurance, and metrology. This accuracy includes the effect of surface roughness, which is minimized by selecting an appropriate diameter of the probing light spot.