Heat treatmen is a process to change metal structure with heated during a certain time, afterward chilled on a cooling medium. Hardening is one of heat treatmen which aims to increase a metal hardness. This study make use of hardening temperature process with 900 ºC by 2 variations holding time, that is holding time with 15 minutes and 30 minutes, then chill using a cooling medium from of water, salt, oil and air. The result of holding time in 15 minutes with water and salt has a hardness value of 61,2 HRA, SAE 20 oil of 57,4 HRA and air of 46,1 HRA. At the holding time of 30 minutes, brine experienced the most significant increase of 68.3 HRA and air of 48.8 HRA, but the SAE 20 oil cooler did not experience a significant change in zinc because SAE 20 oil had a viscosity which caused the specimen cooling rate to slow down.