In this thesis the types of steam boilers and their plants are shown individually, and the strategy of overhauling the steam boiler, which took place in the company "Petrokemija" d.d., is presented. Also, the activities of boiler inspection are described, as well as the testing methods and their implementation, which is carried out after the overhaul is completed. The performance of the overhaul activities is based on a good maintenance organization, adequate equipment, professional staff, tools and an information system. This example of the boiler was overhauled to replace the tube bundle on the pipe rock, and the individual steps are described and explained in detail in this thesis. Maintaining the equipment of the boiler includes various inspection and testing activities, which are undertaken in order to keep the work equipment in a good working condition, with high reliability, safety in operation and availability. This is extremely important for the plant to continue with available and useful work in the company and to eliminate any problems in normal operation, such as accidents.