The high pressure waterjet peening (WJP) was adopted to strengthen the surfaces of 7075-T651 aluminum alloy. The results demonstrate that the processing parameters such as shooting distance, holding time and water pressure affected the abrasive capacity of gas cavities significantly. Compared with non-peening and shot peening (SP) specimens, the fatigue life of as-WJPed specimens was improved by 22% and 6.6%, respectively. The maximum residual compressive stress generated by WJP and SP was approximately equivalent, which is −350 ≤ σ ≤ −400MPa. While the residual compressive stress near the strengthened surface introduced by WJP was higher than that of SP, hence the fatigue life of WJPed specimen increased much more. The surface roughness strengthened by WJP was also better than that of SP. Because the surface profile was well protected in WJP, the good surface qualities also insured a long fatigue life.