The 13^C（a, n）160 reaction is believed to be the main neutron source reaction for the s-process in asymptotic giant branch （AGB） stars. The astrophysical S-factors of this reaction have been determined based on an evaluation of the a spectroscopic factor of the 1/2＋ subthreshold state in 17^O （Ex = 6.356 MeV） by using the 13^C（11^B, 7^Li）17^O a transfer reaction. Our result confirms that the 1/2＋ subthreshold resonance is dominant for the 13^C（a, n）16^O reaction at low energies of astrophysical interest.
