We report the evidence of post-annealing-induced phase transition in zinc phthalocyanine (ZnPc) ultrathin (~20 nm) films upon annealing at 200 °C. The signatures of phase transition are observed in their morphological, structural, and ultrafast spectroscopic properties. X-ray diffraction measurement shows that α-phase nanospheres transform to βphase nanorods that have a high degree of crystallinity with an excellent ordering along the cdirection in a monoclinic arrangement. Tight packing of molecules in the β-crystallites allows a red-shift and intensity gain of the lowest-energy subband in the Q-band and subsequently increases the electronic coupling in both intercolumn and intracolumn paths.