Carbon nanotube fibers with values for specific strength and specific stiffness that are much higher than values reported for any current engineering fibers were reported. The CNT fibers's density was determined using two methods, both of which yielded a consistent value of (0.2±0.01) g cm -3. The first method involved calculating the fiber density from the density of individual CNTs inside the fiber, whereas the second method involved measuring the weight of CNT fibers using a quartz crystal microbalance. The tensile strength and stiffness of the fibers were measured in the range 1.35 to 3.3 GPa and 100 to 263 GPa, using the initial fiber diameter, which was directly measured using a laser diffraction method. The results suggested that the specific strength and specific stiffness properties can be significantly improved by using more longer CNT arrays and by improving the packing density of CNTs in the fiber.