To quantify the relation between process condition, raw materials’ properties, and the interfacial tension of polystyrene (PS)/polypropylene (PP) blend, we evaluated the effect of temperature and molecular weight on the interfacial tension of PS/PP blend with deformed drop retraction method. The accuracy of measurement was improved by eliminating the residual stress in polymers and selecting retraction scale of imbedded PS fiber. Results show that the interfacial tension of PS/PP blends decreases lineally with increasing temperature. The temperature coefficient of PS/PP blends is of the same order of magnitude as reported in the PS/PP systems in the case of similar weight distribution of PS. A primary quantitative relation between interfacial tension, temperature, and molecular weight was established, which could be utilized to guide the design of novel polymer blends.