Diagnostics have been applied to power plant operation monitoring for many years. Much work has been conducted on steam turbine or gas turbine diagnosis with emphasis on vibration or life extending, etc. In recent years, much has been reported on boiler diagnosis, but most has been focusing on thermal parameter monitoring and steam side diagnosis. In combustion research field, deep research work has been carried out on optical measurement and diagnosis technologies. In this paper, we will introduce our recent work on boiler diagnosis with emphasis on combustion fault analysis, measurement methods based on radiant energy of coal combustion, application of artificial intelligence and finally real time diagnosis system development practice.