This study presents a direct numerical simulation of the geometry effect on fluid flow and heat transfer characteristics in the transitional flow over backward and forward facing steps. A computer program of FORTRAN code is used to solve the governing equations according to finite volume method. The effects of the bottom wall length and the step height are investigated. With the increase of bottom wall length, a fundamental frequency of flow fluctuation appears at L-b = 10 h and then disappears at L-b = 14 h. This fundamental fluctuating, frequency decreases with the increase of step height. The fact that the heat transfer performance is better in the reattachment region at L-b = 12 h and S = h indicates that the periodic flow fluctuation with a larger fluctuating velocity has positive effects on heat transfer enhancement. The results of the relationship between flow fluctuation and heat transfer enhancement are significant for certain practical applications of energy conversion. (C) 2017 Elsevier Ltd. All rights reserved.