Steel is one of the most widely used metals in engineering. There are several things that make this material widely used by humans, among others, the amount is quite abundant in nature, usually still in the form of iron ore or pure iron. It has good mechanical properties (strength and ductility), is easy to work with either the casting method or the machining method so that it can be made according to human wishes, and the price is relatively cheap. This study aims to determine the tensile strength and microstructure of ST. 37 steel which undergoes a process carburizing using coal powder with variations of two temperatures, namely 550 °C and 900 °C. The method used in this research is experimental. The first experiment by testing the tensile and the second experiment by observing the microstructure of steel ST. 37, which has been through a heating temperature of 550 °C and 900 °C. The test results showed that the higher the temperature given in the specimen, the greater the value of the pull-yielding, maximum, and fracture that occurs in the specimen. The tensile strength of steel that has undergone a carburizing process is higher than that of steel that has not undergone a carburizing process. The microstructure after the carburizing process shows that the higher the temperature applied during the carburizing process, the atoms contained therein will change according to the given temperature.