alpha-Fe2O3 in three forms of bulk, bare nanoparticles and stabilized nano particles by pectin were used in a fixed bed reactor with changeable diameter of flexible wall and two lamps inside column as a novel design for photocatalytic decolorization of methylene blue (MB) in aqueous solution. At first, the portion of photo degradation as one of the MB removal mechanisms was determined due to comparing performance of reference and experimental reactors. Three parameters of fixed bed reactor containing initial effective diameter of reactor (D-E), inlet flow rate (F) and residence time (tau) were analyzed, experimentally, and the obtained results using three photocatalyst agents were compared. The experimental data was fitted as a function of the independent variables and proposed by the final equations in the terms of coded and actual factors as the linear models for nano alpha-Fe2O/pectin and bare nano alpha-Fe2O, and the quadratic model for bulk alpha-Fe2O3. The most decolorization (C-OMB 100 mgl(-1)) as the experimental and predicted responses were obtained (99.2% and 99.9284%), (98.5% and 99.8849%) and (82.4% and 92.2101%) using nano alpha-Fe2O3/pectin at D-E = 1.7387 cm, tau = 93.073 min and F = 3.219 ml/min, using bare nano alpha-Fe2O3 at D-E = 1.000 cm, tau = 95.125 min and F = 2.033 ml/min, and using bulk alpha-Fe2O3 at D-E = 1.000 cm, tau = 99.999 min and F = 2.000 ml/min, as optimum conditions, respectively. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.