A theoretical analysis for the entrance length of Darcy's flow (i.e.Re≤ 1) into porous media filled in impermeable channels was carried out in the present study. Three channels with different geometries were considered. The entrance lengths were found to be independent of the properties of fluid and porous medium and the Reynolds number as long as the flow is in Darcy's regime. The entrance lengthLeis 1.5R0and 0.75Hfor porous media filled in tubes with radiusR0and 2‐D channels with widthH, respectively. For 2‐D channel flow with jump disturbance at the inlet, the entrance lengths for different disturbance factorsζare obtained and presented. The present results indicate that neglecting this entrance effect could probably result in an overestimation of heat transfer in porous media with lengths in the order of one diameter or one channel width.