The residence time distribution is an important characteristic in the analysis of non-ideal reactors. In micro-reactors that operate predominantly under low Reynolds number, the axial dispersion is important. Since the channel length or residence time of fluid is usually not long enough, the axial dispersion model cannot predict an accurate RTD of the fluid in micro-reactors. In order to overcome this dilemma, a new model has been developed in the present work that combines the axial dispersion and velocity profile using the parallel tanks-in-series compartment model. The model comprises of two parameters, including the velocity profile exponent and Peclet number. It can be used for Newtonian and non-Newtonian fluid flow in the reactors with a wide range of Reynolds number and in all ranges of flow regime, including plug, laminar, perfectly mixed, and other profiles between these regimes. After verifying the model, the effects of velocity profile and Peclet number on the RTD of fluid in the tube for m-laminar and y-laminar velocity profiles were evaluated. Then, the RTDs of some previously reported systems such as straight Polytetrafluoroethylene (PTFE) tube, multi-laminated/elongational flow micro-mixer (MEFM-4) and standard T-junction micro-mixer (TjM) were adjusted to the proposed model. (C) 2018 Elsevier Ltd. All rights reserved.