The considerable portion of energy demand has been satisfied by the combustion of fossil fuel and the consequent CO2 emission was considered as a main cause of global warming. As a technology option for CO2 emission mitigation, absorption process has been used in CO2 capture from large scale emission sources. To set up optimal operating parameters in CO2 absorption and solvent regeneration units are important for the better performance of the whole CO2 absorption plant. Optimal operating parameters are usually selected through a lot of actual operation data. However theoretical approach are also useful because the arbitrary change of process parameters often limited for the stability of process operation. In this paper, a theoretical approach based on vapor-liquid equilibrium was proposed to estimate optimal operating conditions of CO2 absorption process. Two CO2 absorption processes using 12 wt% aqueous NH3 solution and 20 wt% aqueous MEA solution were investigated in this theoretical estimation of optimal operating conditions. The results showed that CO2 loading of rich absorbent should be kept below 0.4 in case of 12 wt% aqueous NH3 solution for CO2 absorption but there was no limitation of CO2 loading in case of 20 wt% aqueous MEA solution for CO2 absorption. The optimal regeneration temperature was determined by theoretical approach based on CO2 loadings of rich and lean absorbent, which determined to satisfy the amount of absorbed CO2. The amount of heating medium at optimal regeneration temperature is also determined to meet the difference of CO2 loading between rich and lean absorbent. It could be confirmed that the theoretical approach, which accurately estimate the optimal regeneration conditions of lab scale CO2 absorption using 12 wt% aqueous NH3 solution could estimate those of 20 wt% aqueous MEA solution and could be used for the design and operation of CO2 absorption process using chemical absorbent.