The strengths of hydrogen bonding interaction between formamide (FA) and thioformamide (TFA) were investigated at the B3LYP level with the 6-311G(d,p), 6-31+G(d,p), and 6-311++G(2d,2p) basis sets. The 18 minimum energy structures of FA-FA, TFA-TFA, and TFA-FA dimers were examined. The average strength of the OCN-H---O=C, SCN-H---S=C, OCN-H---S=C, and SCN-H---O=C hydrogen bonds at the B3LYP/6-311++G(2d,2p) level was -6.1 +/- 0.3, -5.0 +/- 0.1, -4.8 +/- 0.3, and -7.3 +/- 0.4 kcal/mol, respectively, when the basis set superposition error (BSSE) was corrected. The results show that TFA is a good hydrogen bond donor but a poor hydrogen bond acceptor as compared to FA. For the OC-H---O=C, SC-H---S=C, OC-H---S=C, and SC-H---O=C hydrogen bonds, the average strength has been predicted to be -2.2 +/- 0.3, -2.2 +/- 0.2, -1.0 +/- 0.3, and -3.1 +/- 0.3 kcal/mol, respectively. It is remarkable that the thioformyl hydrogen atom of TFA has a strong hydrogen bonding ability as compared to that of FA. The abilities of the hydrogen bond donor have a good correlation with the proton affinities of the deprotonated anion.