Some researchers report that, the HC emission increases due to catalytic coatings in SI engine. The argument proposed by them is the catalysts deplete the air-fuel availability inside the boundary layer by activating them and when the flame reaches the boundary layer, the flame extinguishes due to lack of air-fuel mixture. To investigate the mechanism of flame quenching in a SI engine coated with catalysts, a numerical model of flame quenching phenomena is developed and the results are reported here. Boundary layer thickness and flame quenching distances are calculated. Temperature variations, concentration of species inside the boundary sublayer are also predicted. The model results show that flame quenches due to thermal loss and not by fuel depletion. It has been found that each catalyst has a particular surface temperature above which the surface catalytic reaction increases.