Abstract: A set of micro-system for investigating coking process in transfer line exchanger (TLE) for quenching pyrolysis reaction of gas oil was designed and assembled by the authors. It was clear from the experiment that the coking mechanism in TLE was mass transfer and condensation-surface poly-condensation reaction. The authors first showed a characteristic transition temperature of coking process in TLE. The mass transfer and condensation process could be the controlling step of coking process when the surface temperature was higher than the transition point, otherwise, surface-polycondensa-tion reaction could be the controlling step. On the basis of the above-mentioned mechanism, the authors proposed a coking model of TLE and simulated the industrial TLE. The agreement between the simulation results and observed industrial data was good.