Volatile organic compounds(VOCs) are known to generate ozone by the reaction with NO_x in the atmosphere through a photochemical pathway, which causes the smog. One of the popular treatment processes of VOCs is the thermal oxidation since it shows high destruction and removal efficiency(DRE). Regenerative thermal oxidizer(RTO) is commonly used due to the high heat exchange efficiency. Flow field, temperature, pressure and compositions of flue gas in the RTO were simulated by computational fluid dynamics(CFD) for the steady and unsteady state. The objective of the study is to estimate the DRE of VOCs and the pollutant concentrations of CO and NO emitted from the incinerator with the incomplete combustion and Zeldovich's NO formation mechanism. The case system studied was the combustion of toluene by the RTO composed of three ducts packed with ceramic to exchange heat. The dimension of the duct is 1m × 1m × 1m and height of the oxidizer and ceramic bed is 2m and 0.4m, respectively. Results show that more than 95% of DRE is obtained when the premixed flow velocity of VOC/air is less than 0.6m/s and the average temperature of flue gas is 1,300 K at this condition. It was also found that the inlet velocity is too fast to incinerate VOC/air when the velocity approaches 2m/s. From the unsteady state solution, it takes 2.5 seconds to reach the steady state after exchanging the inlet/outlet duct when the inlet VOC/air velocity is 0.5m/s. Temperature, DRE, concentration of CO₂ and NO were increased with the operating time until 2.5 seconds. The concentration of CO, however, shows the highest value of 1.09%, at 1 second after exchanging duct. It was higher than the steady state concentration of 0.71%. The results revealed that the DRE is lower and the CO concentration is higher than that of the steady state during the transient operating period when the ducts are exchanging.