Abstract: Existing calculation method of relative permeability is based on Darcy’s law. However, studies have shown that flow in low permeability media does not follow Darcy’s law, and that a starting pressure gradient (SPG) occurs in the percolation. Therefore, formulas of relative permeability with SPG taken in-
to account are derived. Additionally, an unsteady state flow wa-ter- oil relative permeability test is performed. The results show that: 1)the residual water and oil saturations are higher in the relative permeability curves of low permeability media; 2)the region of two phase flow is narrower; 3)relative permeabili-ty of oil decreases quickly and relative permeability of water in-
creases slowly with water saturation increasing; 4)oil production declines quickly after water occurs in the wells, and 5)water drive recovery for low permeability reservoirs is lower.

Key words: low permeability pools, non- Darcy flow, unsteady state flow, relative permeability