A theoretical study of the mechanism and kinetics of the inner double hydrogen atom-transfer process in free base porphyrin is presented. Our analysis reveals that the stepwise mechanism first requires the porphyrin ring to compress at an approximate cost of 8.7 kcal/mol, followed by transfer of a H atom with an additional energy requirement of 8.2 kcal/mol. Solvent effects were investigated using a dielectric continuum model and found to be small. The forward and reverse rate constants for the hydrogen atom-transfer process of transisomer <-> cis-isomer were calculated using a canonical variational transition-state theory augmented by multidimensional semiclassical tunneling approximations in the temperature range of 200-1000 K. The calculated activation energy of 10.8 kcal/mol in the temperature range of 200-300 K agrees well with the available experimental data. We found that tunneling is significant for both the forward and reverse trans cis tautomerization processes, especially in the low-temperature range.