It is demonstrated that surface tension diffefence (△γ) can neither act as interfacial ten-sion (γ12) nor as a substituent of γ12 in polymer studies．At present，△γ is still being wrongly used as substituent of γ12 in polymer investigation，because γ12 in polymer system is very diffi—cult to measure．With a purpose to solve the problem，an estimating method of γ12 from the value of breaking energy based on the domain prediction theory of polymer blends developed by the authors is proposed as：γ12=E／3B where B is the experimental constant in domain pre-diction theory; E is macroscopic bulk breaking energy of particle in polymer blends．Using this method，γ12 of any polymer system can be obtained so long as E and B are known．In the case of EPDM，the value of E at 195℃ was found to be 3.3×102J／M3．Besides，two equations based on the modification of Wu’s equation and Girifalco’s & Wu’s equation are proposed to fit some special cases as：When the structure and composition of two polymer materials are almost same．Where γ1 is surface tension of polymer l at temperature l (T1) γ1’is surface tension of polymer l at temperature 2 (T2) γ2 is surface tension of polymer 2 at T1 γ2’is surhce tension of polymer 2 at T2 (T1-T2≈10℃)