The vapor liquid equilibrium of the 3-component system composed of carbon dioxide, water, and methyl diethanolamine has been modeled by the cubic plus association equation of state in a wide range of temperatures (313-433 K), pressures (0.775-4,930 kPa), and methyl diethanolamine wt% (5-75). Carbon dioxide has been considered in two states in this approach: (1) an accumulative molecule bearing the structural shape of 4C and 3B, and (2) a noncumulative molecule. The results obtained from the cubic plus association equation of state showed a good compatibility with the experimental data for the 3-component system. Comparing the results gathered from the Clegg-Pitzer and N-Wilson-NRF models reveals that the cubic plus association model leads to more convincing results than both of them. Furthermore, results obtained from the 4C cumulative design for carbon dioxide in the cubic plus association equation of state shows a lesser error compared to 3B cumulative design and to no designs.