In the present study, gray cast iron was diffusion bonded under air atmosphere and in inert environment (argon). In air it was carried out at the temperatures of 700, 750, 800, 850, and 900 °C under UN pressing load of 1 and 2 ton for 15, 30 and 60min. After diffusion bonding, experimental studies were adopted. As a result, Vickers micro hardness testing was carried out to measure hardness distribution in gray cast iron joint. The results observed from Vickers micro hardness test show that the hardness increases with increasing bonding temperature due to formation of hard phased iron carbides (Fe3C)the maximum value were observed at 850°C for 60 min holding time under 2 ton is 310 HV. The microstructure results show the formation of carbide and the carbide precipitation increases with increasing the bonding temperature and time.