Polyethylene glycol (PEG), as a suitable tool to improve enzyme stability, such as monomethoxylpolyethylene glycol aldehyde (mPEG-ALD) and monomethoxyl-polyethylene glycol succinimide (mPEG-SPA), has been appended at the epsilon-amino group of lysine or the N-terminal alpha-amino acid residue of commercial cellulase. The modified cellulases thus obtained are designated as Cell-ALD and Cell-SPA, respectively. The stabilities and activities of these modified cellulases have been studied in the ionic liquid [C(2)OHmim][OAc]. Cell-ALD showed excellent stability and activity in [C(2)OHmim][OAc], such as the activity of Cell-ALD 5k (molecular weight of ALD is 5000), which can reach above 80% of its original value after remaining in [C(2)OHmim][OAc] for 24 h, and outstanding performance in the hydrolysis of natural cellulose.