Abstract: Experimental investigation was performed for catalyst deactivation by coking in the catalytic cracking process of biomass tar using dolomite and limestone as catalysts. The coke content and tar conversion were monitored during long-term operation. The longest time-on-stream of catalyst is 32 h. It is found that the coke content in the catalyst varies between 3 to 15 percent, and tar conversion drops to about 70 percent. The comparison of coking tendency of tar from different sources was made, and the effect of cracking conditions on coking process was also investigated. The relation between catalyst activity and coke formation was correlated from the viewpoint of kinetics. A linear expression based on the monolayer carbon deposition and an exponential expression based on the multi-layer carbon deposition were obtained separately.