Covalent functionalizations, especially oxidization and derivative functionalization, significantly improve the Surface characteristics of carbon nanotubes, but also bring undesired devastation of sidewalls. This paper presents a theoretic model to investigate double-edge effects of covalent functionalization on carbon nanotubes for the polymer composite application. An optimal degree of functionalization is proposed and calculated based on the trade-off of required load-transfer and induced devastation of nanotube sidewalls. The negative effect on elastic modulus is also calculated. For oxidization-based functionalization of originally defect-free nanotubes, the calculation results indicate that about 10% functionalization degree is suggested to be optimum for the balance of efficient load-transfer and conspicuous devastation from the highly covalent functionalization, The original defects in nanotube surface also reduce the optimum functionalization degree and the reduction is dependent on the original defect concentration. The computational results indicate that nanotube diameter and percentages in the polymer composites show considerably effect on the optimum functionalization degree. Carbon nanotubes with smaller diameter show lower optimum functionalization degree. increasing percentage of carbon nanotubes in the polymer composites also leads to lower optimum functionalization degree. (C) 2009 Elsevier B.V. All rights reserved.