Abstract: In this papery a theoretical analysis is given for non-steady laminar, flow of fluid through flat duct in the entrance region: Starting from rest the analytical explanations have been presented for the velocity distribution. Flow resistance and inlet length are represented by formulas (50), (51) and (52) respectively, The problem of transition from non-steady flow to steady flow is discussed, A particular calculated example is given for the case of average velocity Where is the final value of when steay flow condition is Reached.