A novel fiber-optic sensor structure for noncontact quantification tests on internal MJ threads is presented. It is based on the light reflex measurement principle, but unlike traditional fiber-optic sensors in which the light is emitted from the end face of the fibers, this sensor uses the side cylindrical face, where the converging beam can be obtained. This improves the lateral resolution. The sensor is very compact because a 45 deg, fully reflecting surface is polished at the emitting end of the sensor probe, instead of a fiber-bending structure often used for internal dimension measurement cases. Both theoretical analysis and experimental results indicate the feasibility of measurement of internal threads. Contour of the longitudinal section of an internal thread is measured, based on which shape and parameters of threads can be estimated. The expected accuracy of ∓2 μm for pitch measurement, and ∓10 μm for minor and major diameter measurement have been achieved due to the complex surface shape and measurement conditions.