The far-infrared optical properties of melamine and its deuterated isotope, melamine-d₆ were experimentally and theoretically investigated in the frequency range from 0.2 to 3.0 THz. Under the room temperature and dry air nitrogen conditions, three absorption bands were observed at 2.0, 2.3 and 2.6 THz in the melamine sample by use of terahertz time-domain spectroscopy. Whereas, in the melamine-d₆ sample, the observed absorption bands shift towards lower frequencies and the relative intensity of the absorption bands reduces. Numerical simulation results based on the Parameterized Model number 3 (PM3) were compared with the experimental data and the observed vibration spectra were assigned according to the PM3 calculations. The absorption bands of the measured melamine samples at terahertz frequencies are highly correlated with the intermolecular hydrogen bond stretching and π-π stacking vibration. Also, the red shift of the absorption bands is due to hydrogen/deuterium substitution.