Original TA is hardly dispersed in the polyesters due to their mutual incompatibility. If both were directly blended together, interfacial separation would easily occur in use, resulting in divorce of TA from polyesters. In this work, a tannin grafted polycaprolactone (TA-g-PCL) was synthesized, and its architecture and molecular weight were investigated by FTIR, 1H NMR spectroscopy and gel permeation chromatography (GPC). The influence of molecular weight on the thermodynamics properties and solubility of TA-g-PCL were well determined.