We have performed DFT B3LYP/6-31G(d,p) calculations to investigate the complexation behaviors of the ethyl ester derivative of p-tert-butylcalix[6]arene (1) toward a variety of alkylammonium ions. We have studied the binding sites of these host-guest complexes focusing on the p-tert-butylcalix[6]arene pocket (endo) of 1. The smaller alkylammonium cations have the better complexation efficiency than the bulkier alkylammonium ions with the p-tert-butylcalix[6]aryl ester. The hydrogen-bonding of N-H$ldots$O is one of the important factors for the complexation behavior of the p-tert-butylcalix[6]aryl ester, in addition to the NH-aromatic π, CH-aromatic π and electrostatic interactions, and the steric hindrance of alkylammonium cation. The hydrogen-bonded distances and angles of N-H$ldots$O are reported for the complexes of the p-tert-butylcalix[6]aryl ester with various alkylammonium ions.