By comparing the dynamic interfacial tension (DIFT) of oil/alkali and oil/alkali-surfactant, it was found that, compared with O/A system, the interfacial tension of O/AS system was higher at lower alkaline concentration and lower at higher alkaline concentration. It was also found that OH- played an important role in increasing oil/Alkali interfacial tension by comparing the DIFTmin of Oil/NaOH and oil/NaOH-NaCl under similar ionic concentration. It was also observed that the amount of oil components diffusing into water phase increased with the decrease of interfacial tension. The oil droplet contracted during the measurement of interfacial tension after the oil components left the oil/water interface. Combining all the results together, the synergism between alkali and surfactant for reducing oil/aqueous solution could be deduced as: alkali reacted with oil to form in situ surfactant，added surfactants to stimulate the diffusion of in situ surfactants. Therefore, more surface active components in oil could diffuse to O/W interface, thus producing more in situ surfactants to reduce the oil/water IFT.