Deposition of carbonaceous compounds was used to improve the propylene selectivity of ZSM-5 by deactivating some acid sites meanwhile maintaining the high activity for methanol conversion. The carbonaceous species of pre-coked samples before and after MTP reactions were investigated by elementary analysis and Thermo Gravimetric Analyzer (TGA). The results showed that pre-coke formed at low temperature (250 ℃) was unstable and easily to transform into polyaromatics species at the high reacting temperature. While combing 5% pre-coking process with 95% stream treatment at high temperature (480 ℃) was effective in inhibiting the formation of coke deposits and presented an significant improvement in the propylene selectivity.