In this research, by studying the thermodynamics' laws, the thermal efficiency of microchannel was investigated employing the entropy analysis. Water is used as a base fluid with Al2O3 nanoparticles. Forchheimer -Brinkman model has been applied to include porous zone effect in momentum equations. The two-energy-equation model, which consists of two coupled equations, one for fluid and one for solid phase was employed. The analytical approach is applied to solve energy and momentum equations. The goal for using this method is the existence of nonlinear terms in equations. The effects of magnetic field, Reynolds number, alpha and epsilon on the parameters affecting irreversibility were studied. Then, it was concluded that by augmenting the Reynolds number to Re-op, N-s decreases. Beyond Re-op, the rise in Re enhance the S-gen and declines efficiency of the system. However, applying the magnetic field is beneficial between Re = 1.1 and Re-cr, and reduces the generated entropy. It was also observed that with the increase of alpha, N-s takes an upward trend. Increasing the epsilon value from 0.25 to 0.5 results in the downward trend of N-s, however, in high epsilon (0.75), N-s rises noticeably with augment of frictional irreversibility. (C) 2019 Elsevier B.V. All rights reserved.