Cotton stalk was pretreated with 1-butyl-3-methyimidazolium chloride ([Bmim]Cl) and the effect of pretreatment factors, including liquid/solid ratio, temperature, and pretreatment duration on cotton stalk was investigated. Scanning electronic microscopy, Fourier transform infrared spectroscopy, and Xray diffraction were used to identify structural changes in native and pretreated biomass. The optimum pretreatment conditions were as follows: liquid/solid ratio of 10:1, reaction temperature of 120 degrees C and duration of 5 h. The reducing sugar yield was 29.72%, which was significantly higher than that of native cotton stalk. The differences in the reducing sugar yields between [Bmim]Cl-pretreated and untreated biomass suggested that [Bmim]Cl pretreatment was effective to produce reducing sugars. The pretreated cotton stalk showed clear pores and cracks, low crystallinity, and incomplete crystal structure compared with untreated one. [Bmim]Cl could be reused at least four times without significant reduction in the reducing sugar yield. (C) 2018 Elsevier Ltd. All rights reserved.