The surface characteristics and microstructure of a 100-oriented single crystal superalloy subjected to ultrasonic shot peening (USP) were investigated as a function of peening duration. The obtained results show that USP significantly increases the surface microhardness of the single crystal superalloy, reaching up to 650 HV. As peening duration increases, surface roughness and microhardness increase at first and then remain stable. Meanwhile, new components and orientations are formed on the treated surface. The depths of the cold work layer and slip band reach saturation with sufficient peening durations. However, the microstructure beneath the surface continues to evolve with increasing peening time. Excessive peening duration will facilitate the formation of curved slip bands, distinct misorientation and subgrains, which may damage the coherent strengthening of γ/γ′ structures and result in a lower hardness value in the depth direction. Combining the effects of surface alloying, strain-induced diffusion and strain-induced dissolution, four layers (i.e., the nanocrystalline α-Fe layer, Fe, Ti, Ni interdiffusion layer, nickel-based γ′-free layer and deformed γ/γ′ layer) are formed on a single crystal substrate with sufficient USP durations. For single crystal superalloys, there exists an optimal processing duration to achieve the balance between surface hardening and coherent strengthening of γ/γ′ structures. Display omitted •A 100-oriented SX superalloy was ultrasonic shot peened with 2–60 min.•With peening duration increase, deformed layer depth is saturated, while microstructures continue to evolve.•Optimum duration is 2–4 min for SX to enhance the surface hardness and keep γ/γ′ structures integrity.•Excessive duration will damage the γ/γ′ structures, cause distinct misorientations and lead to the lower hardness with depth.•By USP, nanocrystalline α-Fe, interdiffusion, Ni-based γ′-free and deformed γ/γ′ layers are formed on SX substrate.