Introducing substituents onto SWNT sidewalls increases their solubility and tunes their properties. Controlling the degree of functionalization is important because the addition of numerous functional groups on the sidewall degrades their intrinsic useful electronic properties. We examined the synthesis and characterization of sidewall-functionalized SWNTs in this study. The functionalized SWNTs (R-1-SWNTs-R-2) were prepared in a one-pot reaction of SWNTs with alkyllithium ((RLi)-R-1) followed by alkyl bromide ((RBr)-R-2). The functionalized SWNTs were characterized by the absorption and Raman spectroscopy and thermogravimetric analysis. Not only the total number of functional groups introduced on the SWNT sidewall (formula mass: R-1 = R-2) but also the ratio of R-2 to R-1 in the fimctionalized SWNTs (formula mass: R-1 not equal R-2) having two different substituents were clarified using the relation between results of Raman spectroscopy and thermogravimetric analysis. Results show that the degree of functionalization of R-2 to R-1 in R-1-SWNTs-R-2 can be well controlled by the bulkiness of the alkyl groups of (RLi)-R-1 and (RBr)-R-2. Moreover, substituent effects of reductive alkylation and reductive silylation of SWNTs via Birch reduction were investigated.