Abstract: The interracial tension of water/kerosene with added AP type polyoxyethyle- ne-polyoxypropylene block copolymers has been determined, as a function of con- centration of copolymer at 298 K and as a function of temperrature at fixed con- tent of copolymer. The result shows that the AP type polyoxyethylene-polyoxypro- pylene block copolymers exhibit a high interracial activity. The effects of EO con- tent and PO distribution in the AP type copolymers have been investigated. At low concentrations a linear decrease of lnγ as increasing of ln(c+1) was observed. The interfacial tension decreases with increasing temperature sharply. Primary experiments indicate that the AP type copolymers are very effective demulsifiers. The relation between interfacial tension and demulsification effect on several sources of crude oils has also been discussed.