The microstructural role of TiB addition in modifying a Zn-Al eutectoid (ZA27) alloy was investigated using scanning electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and energy X-ray spectroscopy. A series of Zn-27Al-2Cu-0.02Mg-x(Ti+B) (x=0, 0.01, 0.02, 0.05, 0.10, 0.20, wt.%) alloys were cast. The results show that the hardness and strength of the ZA27 alloy are improved with increasing (Ti+B) content, and peak at a (Ti+B) content of 0.05 wt.%. The TiB modifier refines the primary α(Al) phase grains and changes the microstructure of coralloid (α(Al)+η-Zn) eutectoid, which is responsible for the increased mechanical properties of the ZA27 alloy. The modified eutectoid demonstrates featured basket-like and strip-like eutectoid microstructures. The refinement of the primary α(Al) phase grains is very likely related to the constitutional supercooling caused by the segregation of B at the solid-liquid interface. Participation of B and/or Ti in the eutectic and eutectoid reactions modifies the morphology of the eutectoid microstructure.