Abstract: The variable and micro effect of pore size make fluid flow more complex in low permeability reservoirs,which caused non-linear fluid flow and starting pressure gradient,laboratory experiments of low permeability core have been done on block 26 of Shengli Oilfield 74 block Dongying group,measured its starting pressure gradient of single phase and oil-water two phases flow,the experiment result showed that at single flow of oil,while the displacement fluid mobility reached to a special value,the starting pressure gradient decrease gradually and smoothly with increase of mobility,and the effect of mobility to starting pressure gradient is evident.At different initial water saturation of two phases flow,the starting pressure gradient and mobility increase with exponential function,and the starting pressure gradient decrease with the increase of water saturation.Weighted to saturation based on viscosity of mixed oil and water fluid,established unified experiment model of starting pressure gradient,which is suit for single and two phases flow,and has preferable self isolate.