Effective utilization of carbon nanotubes (CNTs) as reinforcements in composites necessitates good interfacial bonding with the surround matrix material. The covalent functionalization of CNTs is an effective method to enhance this bonding. However, covalent bonds introduced by a functional group may alter the pristine structure of the CNT and lower its mechanical properties. Here we study the effect of hydrogen (–H), hydroxyl (–OH), carboxyl (–COOH), and amine (–NH2) functionalization on Young’s modulus of a single-wall CNT (SWCNT) using molecular mechanics (MM) simulations with the MM3 potential and the software TINKER. Both pristine and functionalized SWCNTs have been deformed in simple tension. From the strain energy of deformation vs. the axial strain curves, the value of Young’s modulus has been derived as a function of the functionalization group and the amount of functionalization. It is found that Young’s modulus decreases by about 30 % with 20 % of functionalization, the reduction is essentially proportional to the increase in the percentage of the functionalization material and is nearly the same for each of the four functional groups studied.