Abstract: The characteristic equations of micro forces such as van der Waals force, electrostatic force, steric type force and surface tension were established on the basis of the existing experiments and theories of micro flow. The effect of various micro forces on fluid flow in porous media was analyzed in detail. A mathematical model considering the effect of micro forces was constructed for fluid flow in micro tubes. A new relative permeability formula considering micro forces was derived using the capillary bundle model of porous media. Numerical analysis clarified the effect of micro forces on fluid flow in porous media. Simulation results show that micro forces cannot be neglected in micro-scale porous throats.