Abstract: Cotton bast fiber treated with acetic anhydride was used as a reinforcer of polypropylene forming composite sheet of 4 mm thick， when the content of cotton bast fiber was 30 wt%. The cotton bast fiber was treated in a NaOH solution, and subsequently in a bath of acetic anhydride for 30 min at 30 ℃. The composite sheet acquired excellent tensile and bend strengths， increased by 6.33 % and 11.72 %, respectively.