Co-firing of Russian caking coal with non-caking coals (the mixture of Australian bituminous and Korean anthracite coal) was carried out in a pilot scale circulating fluidized bed combustor (CFBC). The free swelling index (FSI) of the caking coal was 6.5 and the alkali index of coal blends ranged from 0.39 to 0.51. The stable fluidization and combustion, however, were achieved with the coal blends (up to 20 wt.% of strongly caking coal). The average particle size of the bottom ash collected after tests increased, causing the slight temperature rise in the lower part of the combustor. SEM/EDS and XRD analyses of coarse particles showed that the Al-rich bottom ash captured problematic K and formed kalsilite (KAlSiO4) compound. Its melting point is much higher than the operating temperature of the typical CFBC. This strongly supports that the stable co-combustion is possible in the CFBC by using the mixture of strongly caking coal. The unburnt carbon content in the fly ash and O2 concentration in the flue gas obtained during the steady state condition increased linearly with an increase in the content of caking coal, indicating the restriction of the co-combustion ratio of the caking coal. Morphology analyses of the unburnt carbon in the bottom ash recovered after experiments revealed that the pore clogging progressed with the addition of the caking coal, resulting in the decrease of the char reactivity by blocking oxygen access during the co-combustion. Regarding emission, NO emission in the flue gas from fuel N was slightly increased but that of N2O decreased slightly, due to the temperature rise in the dense bed of the combustor. However, the conversion rate from fuel S to SO2 did not change considerably with increasing the content of caking coal.