The large amount of dye wastewater requires efficient techniques, e.g., adsorption by activated carbon (AC), for removal of dyes to decrease water pollution. AC was modified by the basic ionic liquid 1-butyl-3-methylimidazolium hydroxide. The surface functional groups of AC changed evidently as detected by Fourier transform infrared spectroscopy and Boehm titration. To evaluate the influence of functional group change on adsorption performance, AC and modified activated carbon (MAC) were employed to remove methylene blue. The results indicated an increase of adsorbance with lapse of contact time. The adsorption capability of AC was distinctly higher than that of MAC. From kinetic studies it follows that the pseudo-second-order kinetic model can well describe the adsorption process.