CO2 absorption by ammonia added triethanolamine aqueous solution as a promoter was investigated in terms of absorption percentage (AP), overall volumetric mass transfer coefficient (KGae), and molar flux (NA) in a packed column. Three variables of ammonia concentration (0–5 wt%), Triethanolamine concentration (10–30 wt%), and gas flow rate (1,500–2,500 ml/min) were considered as significant variables in absorption performance. Effect of these variables and their interactions were inspected using the three level factorial response-surface method. Statistical analysis of the results showed that an ammonia concentration with 72.99%, 71.83, and 81.12% has the greatest effect on AP%, NA, and KGae, respectively. Then, gas flow rate with 5.27% and 3.90%, had a great effect on AP% and KGae, respectively. Finally, the optimal operating conditions were determined to maximize the responses. Under optimal operating conditions, the maximum values for AP%, KGae, and NA were 98.94%, 0.202 kmol/h·m3·kPa, and 3.901 kmol/m2·h, respectively. Thus, adding ammonia to triethanolamine considerably improves the mass transfer performance of solvent.