We calculate vibronic coupling constants (VCCs) and reduced vibronic coupling densities (RVCDs) of free-base porphin (FBP) cation whose derivatives are employed as hole-transporting molecules in organic field-effect transistors (OFETs). The FBP cation has small VCCs as a p-conjugated molecule. Vibronic coupling density (VCD) analysis reveals that strong localization of the electron-density difference on the meso-carbon atoms between the cationic and neutral states leads to the small VCCs of the FBP cation. This study is a starting point for understanding vibronic couplings in a hole-transport process through porphyrin thin films. (C) 2011 Elsevier B.V. All rights reserved.