Abstract: This study investigates the effects of two spot sizes (200μm and 800μm) on the microstructure and microhardness of laser welded joints of 304L stainless steel and Inconel 600 alloy. The results show that the 200μm small spot, with its high energy density of 6.37×10¹²W/cm², produces a narrow and deep weld with a depth-to-width ratio of 0.81, which is characteristic of the keyhole welding mode. The extremely rapid cooling rate results in a fine microstructure dominated by fine dendrites and cellular crystals. In contrast, the 800μm large spot, with a lower energy density of 3.98×10¹¹W/cm², forms a wide and shallow bowl-shaped weld with a depth-to-width ratio of only 0.39, which is indicative of the conduction welding mode. The slower cooling rate leads to exceptionally coarse grains and a distinct unmixed zone. Hardness softening occurred in both weld metals. However, the large-spot weld exhibited a higher average hardness of 83.0HV compared to 72.8HV for the small-spot weld, suggesting that slower cooling promotes compositional homogenization and partially mitigates softening. Nevertheless, the small-spot weld contained numerous inter-dendritic precipitates and micro-cracks, while the large-spot weld showed increased embrittlement tendencies due to coarse grain boundaries and interfacial brittle phase formation. This study confirms that spot size is a critical parameter for controlling welding mode, solidification microstructure, and mechanical properties.