Paper mill using recycle fiber as raw material produces reject containing fiber and plastic. The reject was identified by means of sorting. Reject pellet was prepared by drying, followed by shredding, and pellets moldings. It was analyzed for its proximate composition (moisture, ash, volatile matter and fixed carbon). Calorific value, sulfur content, ash mineral content, and ash fusion temperatures were also tested. The results showed that reject consists of 50.75% fiber and 49.25% plastic of which is >99% high density polyenthylene (HDPE). Reject contains a high calorific value of about 7000 cal/g which could potentially be used as fuel. Coal boiler fuel containing 5-50% reject pellet shows no indication of slagging and fouling in the boiler.