A new technique for measurement of the apex angle of a right-angle prism is proposed. A reference artefact is newly employed as the datum for angle measurement, and the apex angle of a target prism is measured by detecting its deviation from that of the reference artefact by using a pair of optical angle sensors. At first, the principle of the apex angle measurement with the reference artefact and the optical angle sensors are addressed. After that, an optical angle sensor capable of evaluating various sizes of right-angle prisms over a large measurement range is designed and developed. A two-dimensional position-sensitive detector is employed as the photodetector for the optical angle sensor so that a high resolution and a wide measurement range can be achieved simultaneously. To avoid the influence of the interference between a directly-reflected beam and an internally-reflected beam when measuring a face of a right-angle prism, the optical setup is designed to have a laser beam that is made incident to a measurement surface with an oblique incidence. After the design and development of the optical angle sensor, the measurement system has been developed and some basic experiments are carried out to evaluate the basic performance of the developed optical angle sensor. The feasibility of the proposed technique of the prism apex angle measurement is also demonstrated through experiments and theoretical measurement uncertainty analysis based on GUM.