We have studied the effect of the sulfonate concentration on the interfacial tension between Dagang crude oil and solutions of a mixture of Daqing and Kelamayi petroleum sulfonates (DPS and KPS). We found that a flooding system containing 0.05% DPS and 0.05% KPS can reduce the interfacial tension down to less than 10-5 N/m in the presence of polymer, without using alkali and alcohol. Injection of 30% PV (pore volume) of a slug of this flooding system followed by chase water resulted in a 17.40% increase in oil recovery compared with recovery by waterflooding alone. The final oil recovery was as high as 79.55% of the proven oil reserve.