This work aims to investigate the effects of particle size on flow regimes and validation of Darcy law. Experiments were conducted in sand columns with five different particle sizes of silica sand in mean diameter 1.075 mm, 1.475 mm, 1.85 mm, 2.5 mm and 3.17 mm. Both pre-Darcy flow and post Darcy flow were identified but no obvious linear flow was observed, the Reynolds number of two flow regime are different with different grain sizes. Relative error was used to evaluate the experimental data and obtain that the values of Reynolds number at the demarcation point are 3.90, 7.08, 9.1 and 10.78. The values of Reynolds number of two flow regime increased gradually as the particle diameter increased. A new Reynolds number defined as ratio of inertia term and viscous term in Forchheimer equation can lead to a single criterion for determining the limit of validity of Darcy law. The recommended critical Reynolds number for non-Darcy flow is equal to 0.1, where the error of neglecting inertial effects in the hydraulic gradient will be less than 10%. (C) 2016 Elsevier Ltd. All rights reserved.