The emission control of motorcycles in Vietnam is particularly important due to the high and fast raising number of these means of transport. One of the most effective method sused for emission control of internal combustion engines is exhaust after-treatment, which has not been used in small capacity motorcycles here. This paper presents the study results on heat transfer and catalytic conversion modelling of the exhaust gas in motorcycle exhaust after-treatment system equipped with a three-way catalytic converter for optimum design of exhaust emissions control system. The paper considers the chemical conversion of carbon monoxide (CO), unburned hydrocarbons (HC) and oxide of nitrogen (NO) with detailed heat transfer modelling, which includes conductive, convective and radiative heat transfers. The modelling results show that using exhaust after-treatment system equipped with a three-way catalytic converter for emission control of motorcycles can reduce exhaust emissions of NO and CO by 99 percent and HC by 70 percent after 300s from engine start.