In our current era, each and every field are running and upgrading to automation. Without electricity, automation is not possible. In India, electricity production is mainly of coal based thermal power plants. Efficiency of a conventional coal based power plant varies from 34-38%. Efficiency of the plant should be maintained and improved in order to use the fossil fuels effectively. Efficiency of a thermal power plant mainly depends upon the boiler efficiency and turbine efficiency. This paper is about determination of boiler efficiency of a 510MW thermal power plant. This paper presents the efficiency calculation of boiler. The study focuses on proximate analysis and ultimate analysis and various parameters like theoretical air supply, excess air, actual air supply, dry flue gas loss, loss due to H2, loss due to moisture, loss due to un-burnt carbon in fly and bottom ash, loss due to radiation and convection, etc. comparison of the design and operational values are included. Justification and various parameters are mentioned for further improvement of plant performance.