In the present work 2D versus 3D numerical simulations of mixing phenomenon in a micro-channel has been compared. The blockage ratio of the barriers in micro-mixers has been changed and it was observed that when the blockage ratio of the barriers against the flow direction was close to 1 or zero, the results of 2D simulations approach to those of the 3D cases. On the other hand, there was a significant inconsistency between 2D and 3D mixing indices when the blockage ratio was about 0.75. For the micro-barriers attached to upper wall, at middle Reynolds numbers (2 < Re < 10) 2D mixing indices were more close to 3D mixing indices. For blockage ratios of 0.5, the presence of vortices induced by the secondary flow in the z-direction was the main reason for higher mixing indices and the consequent deviation of 2D versus 3D results. The lateral Peclet number for the transversal mass transfer was also studied. (C) 2019 Elsevier Ltd. All rights reserved.