From last epoch till to date, AISI 4340 and AISI 4140 have been widely used in different engineering applications. These applications include bolt, screws, gears, drive shafts, crane shaft and piston rods for engines due to its upright mechanical properties, cost-effective and easily available in market. In present work, deep cryogenic treatment effect on the mechanical properties of AISI 4340 and AISI 4140 have been studied. Present work was carried out at laboratory scale and can be extended for mass production. Our work is simple, straight forward safe and economical. In our work, samples were heat treated in simple muffle furnace and followed by cryogenic treatment in liquid nitrogen. Before cryogenic treatment, all samples were normalized at 860°C to obtain homogenized... microstructure. Samples were also compared conventionally heat treatment with quenched in oil quenchant. Experimental results showed that after cryogenic treatment with tempering treatment, one could easily increase the tensile strength, impact toughness and hardness. Advanced optical microscopy (IMM 901) and SEM (Scanning Electron Microscopy), FIT Quanta 200 methods have also been deployed to reveal and interpret the internal structure of samples. It was found from microstructure that cryogenic treated sample increases the impact strength, hardness and tensile strength as compared conventional heat treated quenching approaches.