We studied the complexation of cyclo (L-Pro) 3 with alkali metal cations (Li + , Na + , K + , Rb + and Cs + ) in the gas phase using density functional theory (DFT) calculations. The complexes were optimized at B3LYP/6-31+G(d) and CAM-B3LYP/6-31+G(d) levels of theory. The binding energy of M + - cyclo (L-Pro) 3 complexes was increased in the following order: Li + > Na + > K + > Rb + > Cs + . Natural bond orbital (NBO) analysis at B3LYP/6-31+G (d) level was performed on the optimized geometries. These results indicated that the complexation in M + - cyclo (L-Pro) 3 complexes, was caused by the lone pair electrons of electron donating oxygen atoms and the LP* orbitals of alkali cations. The electron density at bond critical points was consistent with the binding energy of M + - cyclo (L-Pro) 3 complexes.