The interaction between carbon nanotubes and organic molecules including benzene (C6H6), cyclohexane (C6H12), and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ: C8N2O2Cl2) have been studied using first principles calculations. The equilibrium tube-molecule distance, adsorptionenergy, and charge transfer are obtained. The hybridization between the DDQ molecular level and nanotubevalence bands transforms the semiconducting tube into a metallic one. Coupling of π electrons between tubes and aromatic molecules are observed. Our results show that noncovalent functionalization of carbon nanotubes by aromatic molecules is an efficient way to control the electronic properties of carbon nanotubes.