The laser peening is a surface treatment technology that improves the mechanical performance of metals. For instance, it has been utilized to increase the resistance of a metal to crack initiation, extend the fatigue life, and enhance the fatigue strength. These effects are imparted by shockwave generated by the expansion of laser-induced ablation plasma. Utilizing double-pulse laser ablation is one of the methods to improve a coupling of the laser beam with the plasma plume and target material. With double-pulse laser ablation, a target material is irradiated by two pulsed laser beams separated by a nanosecond inter-pulse delay time. It is reported that this irradiation method allows for a more effective energy delivery to the plasma plume and target material. In this study, double-pulse laser ablation has been adopted for laser peening. The experiments were performed by two Nd: YAG Q-switched pulsed lasers (wavelength: 532 nm) in water ambient. Austenitic stainless steel SUS316L was used as the target sample. The pulse durations of the first pulse and second pulse were 6 and 4 ns, respectively. Varying the inter-pulse delay time, the surface hardness on the sample after laser peening was evaluated. The results indicated that the surface hardness depends on the inter-pulse delay time. It is found that optimizing the inter-pulse delay time for a double-pulse laser enables enhancement of the laser peening effect.