Novel miscible ternary blends comprising poly(ethylene azelate) (PEAz) and poly(ε-caprolactone) (PCL), which are biodegradable polymers, and catechin, a natural polyphenol, were discovered. The inherent biodegradability of PEAz and PCL and the biocompatibility of catechin enable these blends to be used for manufacturing functional polymeric materials. The PEAz/catechin/PCL blends exhibit homogeneous phase morphology and one single T g for each of the ternary compositions, demonstrating that the blends are with ternary miscibility. The phenomenon of melting point depression found in the blends confirmed that the PEAz/catechin/PCL blends are inherently miscible with thermodynamically favorable interactions. It also revealed that catechin formed hydrogen-bonding interactions with the polyesters PEAz and PCL in the ternary blends. Ternary miscibility in the PEAz/catechin/PCL blends might be mainly attributed to two factors: (1) a negligible ΔK effect resulting from the symmetric hydrogen-bonding interactions between catechin and PEAZ and between catechin and PCL, and (2) a minimal Δχ effect caused by a slight difference in the solubility parameters of PEAz and PCL in the blends. In these miscible ternary blends, the nonisothermal crystallization of crystalline polyesters was retarded as the catechin content was increased.