Due to escalating cost of fuel, there is a felt need to carry out efficiency test to analyse the various heat losses present in boiler, with an aim to identify the major heat losses and causes of poor boiler efficiency. In this book, boiler efficiency improvement study has been carried out in a large boiler of a pulp and paper mill. The causes of poor boiler efficiency were various heat losses like loss due to unburnt carbon in refuse, loss due to dry flue gas, loss due to moisture in fuel, loss due to radiation , loss due to blow down and loss due to burning hydrogen. The various heat loses were analysed and certain recommendations were made to the plant management for implementing, so that efficiency of boiler can be increased. Some of the recommendations made were implemented and it has been seen that there is tremendous increase in boiler efficiency.