A fiber optic rotary joint is invented using TEC (Thermally Expanded Core) fiber. This device has a rotatable optic TEC fiber and a stationary optic TEC fiber in an assembly to convey a light beam in the optic TEC fibers. The assembly also includes a stationary optic TEC fiber holder for fixing the tip of stationary optic TEC fiber in a central hole and a rotatable optic TEC fiber holder for fixing the body of the rotatable optic TEC fiber with the tip of the rotatable optic TEC fiber protruding out of the rotatable optic TEC fiber holder. The two optic TEC fiber holders can be rotated relative to each other so as to allow the tip of rotatable optic TEC fiber to rotate in the central hole on the stationary optic TEC fiber holder with the facets of the two optic TEC fiber adjacent very closely.