It is well-known that carbon nanotubes (CNTs) have unique properties: thermal conductivity of 3000 W/m·K, electrical resistivity of (5 to 50) μΩ·cm, and tensile strength of about 50 GPa [1-4]. These promising properties have attracted the interest of researchers studying CNT-polymer composites in a variety of fields including electromagnetic wave shielding, heat spreaders, and high strength structures [5-9]. Despite this effort, there has been limited improvement in the thermal conductivity and mechanical properties of composite materials. One of the most fundamental problems in improving the properties of CNT-based composites has been controlling the CNT diameter and length. Change in the diameter of carbon nanotubes directly affects CNT characteristics, and CNTs with larger diameters are known to have increased defect density and diminished CNT characteristics [10]. Kis and Zettl [11] found that the Young's modulus of CNT decreases with decreasing particle diameter. Fujii et al. [4] experimentally demonstrated that the thermal conductivity of CNTs decreased rapidly from (3000 to 500) W/m∙K when the diameter increased from (8 to 30) nm.