The objective of this study is to investigate whether the PEGylated conjugated linoleic acid (PCLA) as an anti-cancer prodrug can have favorable stability, biological activity, and prevention of proliferation in MCF-7 breast cancer cells for anti-cancer when compared with conjugated linoleic acid (CLA) itself. The CLA was simply coupled to poly(ethylene glycol) (PEG) at melting state without solvent or catalyst through ester linkage between carboxylic group of CLA and hydroxyl one of PEG. The results showed that the half life of PCLA was 55h in cell culture medium at pH 7.4 and 37°C. Apoptosis of MCF-7 breast cancer cells were induced by not only CLA- but PCLA-treatment with increasing concentrations whereas PCLA increased cell viability when compared with CLA itself. These results indicate that the PCLA is a more stable and valuable prodrug in that it has good stability and inhibition of cancer cell proliferation.