Stability of coke size is a prominent index for evaluating the degradation of coke during transportation and inside blast furnace. The coke with high stability, showing higher broken-resistance and abrasion resistance, are good for high PC injection operation. A method for evaluating the stability of coke by using the Micum tumbler was developed in this study. Three different group of sample that produced at phase II and III coke oven were compared with the stability indices. The result shows that the cokes produced at phase III have the highest index while the cokes from the by-pass process at phase II have the lowest one.