Co-combustion testing of Low-Rank Coal (LRC) and Medium-Rank Coal (MRC) was carried out in a Circulating Fluidized Bed (CFB) Boiler with a capacity of 16.5 MWe to determine the effects on the performance and characteristics of operational boiler parameters. Coal was initially divided in two different bunkers based on its quality. The big bunker was filled with LRC and the smaller one was filled with MRC. To observe further impact on boiler performance, four different flow ratio percentages were set up through the coal feeder. We found that increasing the MRC ratio contributed to the rising furnace bed and inlet cyclone temperature. This also positively affected the boiler efficiency with the highest value of 86.77 % with full of MRC. However, the co-combustion caused drawbacks due to inhomogeneous temperature distribution within those furnaces. The average bed temperature ranged from 974 0C to 1,000 0C. The Net Plant Heat Rate (NPHR) increased slightly by 3.95 % of the original fuel design.