The objective of this study was to propose a facile method to manipulate the molecular weight and practical mass production of chitosan by mechanical shearing and concurrent ultrafiltration (UF) treatment. The proposed method was based on the degradation rate and rate constant of various process variables, such as: solution temperature, reaction time, concentration of chitosan solution, with or without concurrent removal of degraded fragments during mechanical shearing. The result obtained was that the degradation rate constant was 1.8-6.0 times higher for those using UF to remove smaller degraded molecules concurrently during treatment, than that without UF treatment. The degradation rate constant increased with increasing solution temperature; however, the solution temperature should not exceed than 50 degrees C to prevent the undesired color changes of the resulting product. A method combining mechanical shearing/UF treatment at 50 degrees C and ultrasonic radiation or microfluidization/UF treatment at 30 degrees C is proposed here for a facile method to manipulate the molecular weight of the resultant chitosan with an energy saving, efficient and practical mass production. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 1442-1449, 2010