Compressors are machines which compresses gas on to higher value of pressure respectively they raise its energetic value. That is sort of compressed air that is used to put marine diesel engine into work. The most used sort of the compressors on ships are two-stage reciprocating compressors. From thermodynamics of compression we look at polytropic process, isothermal process and isentropic process. Multistage compression is used to save power and for higher stage of compression, compressor is closer to isothermal compression. Multistage compressor heats air in compression process, which can lead to explosion of the steam of lubricating oil mixed with air. To compress air to high pressures and to avoid overheating and danger of explosion it is used multistage compression with intermediate stage cooling. Compressors on ships are mostly driven by electromotors. It should be taken into account that during going in to full speed we don’t overload electromotor. Compressor and all his components are needed to maintain regularly in periods which depends on several working conditions. In today modern days compressors in start air system are automatically regulated