An optical fiber assembly includes a prism, a light emitting element, an optical fiber, and a light detector. The prism includes an incident surface, an emergent surface, a first reflecting surface, a second reflecting surface, and a third reflecting surface. A collimating portion is positioned on the incident surface. The light emitting element faces the collimating portion for emitting light beams. The optical fiber faces the emergent surface. The light detector faces the incident surface. The collimating portion collimates the light beams to parallel first and second light beams. The first light beams are projected to the first reflecting surface, reflected to the emergent surface, and are reflected to the fiber optical. The second light beams are projected to the second reflecting surface, reflected to the third reflecting surface, and reflected to the light detector.