A novel method to synthesize alkylated chitosan with [Bmim]OH basic ionic liquid as alkaline reagent is reported for the first time. Chitosan was alkalized by [Bmim]OH basic ionic liquid before it was reacted with various alkyl halides, and a series of alkylated chitosans with different carbon chain substituents were prepared. The structure of alkylated product was characterized by IR, 1H-NMR, and XRD. The reusability of the ionic liquid was also investigated, and the experimental results indicated that there was no noticeable change for the degree of substitution (DS) of alkylated chitosan after the ionic liquid was reused for three times. The antibacterial activities of the target products were investigated according their minimum inhibitory concentration (MIC) against K. pneumoniae, P. aeruginosa, E. aerogenes, E. coil, S. epidermidis, and S. aureus, and the results indicated that E-CTS, B-CTS, and D-CTS could exhibit excellent antibacterial activities against P. aeruginosa, and their MIC were 128, 128, and 256 μg mL−1, respectively. Meanwhile, the antibacterial activities of alkylted chitosans were decreased with the growth of the DS or the growth of the carbon chain. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40052.