In this study, the AISI 420 was laser surface transformation hardened (LSTH) utilizing a high power diode laser (HPDL). The experimental tests were performed based on design of experiment method. Laser power, laser scanning speed, and focal plane position (FPP) of laser beam were considered as input parameters and their influences on geometry of the hardened area (depth and width), the maximum micro-hardness and the hardness deviation from the base metal were investigated. The laser hardened samples were structurally analyzed by metallurgical investigation and mechanically evaluated by micro-hardness testing; Microstructural studies on laser-treated surfaces were conducted by scanning electron microscopy (FE-SEM) and optical microscope. The micro-hardness of the width and depth of the hardened layer was measured. Macro-image testing has been done to measure the hardened area geometric dimensions accurately. After the aforementioned tests, the interpretation of how the various parameters of the laser are affected by the quantum exit of the components is discussed. Result revealed that when the laser power raises and laser speed fall off, dimension of hardness area were increased. When FPP decreased, hardened depth was decreased and width was raised. Statistical analysis of the results was performed using the Design Expert software to explore the penetration of mechanism parameters on the behavior of responses, to obtain regression equations and to predict the results. Optimization of the LSTH process to achieve an optimal hardness and also optimal settings of parameters were the other goals of this exploration.