Abstract: This paper introduces ways to determine technically rational well density in a low porosity and low permeabili- ty reservoir based on factors such as relationship between well density and waterflooding reserves control level, ulti-mate recovery factor, rate of oil production, displacement pressure gradient effective permeability and investigation radius; presents methods to select economically optimal well density and economy limit well density. An optimization principle of rational well density is to realize the largest economic benefits while meeting the fundamental require- ments of development of low porosity and low permeability reservoirs. When technically rational well density is larger than economically optimal well density while smaller than economic limit well density, the former should be selected. Calculation results show that this method is practical.