The polarization-maintaining (PM) capability of a high- birefringence (hi-bi) optical fiber can be described in terms of its modal birefringence or beat length. In the hi-bi (PM) fiber the beat length can be measured by changing the state of polarization of the light through an external means. One such technique is based on the Faraday magnetooptic effect. The technique is nondestructive, relatively simple, and efficient, and beat lengths of 2 mm and less have been measured accurately with it, thus fulfilling the measurement requirements for many of the currently available PM fibers. Utilizing optical sources with wavelengths of 633 and 814 nm, the beat lengths in a total of four hi-bi fibers have been successfully measured using the dc Faraday magneto-optic effect. The experimental results are comprehensive and very promising, and it has been demonstrated that this nondestructive technique has significant advantages over many others in simplicity, versatility, productivity, practicality, and accuracy.