Hydraulic fracture is important in unconventional oil and gas exploration. During the propagation of the hydraulic fracture, the crack tip is blunted due to the development of the process zone in the near-tip area. In this study, the blunting of the hydraulic fracture in polymethyl methacrylate specimens due to multi-timescale stress concentration is investigated. The ratio of the initiation toughness to the arrest toughness of the blunted hydraulic fracture is measured using both the dynamic and the static methods. Results show that a hydraulic fracture can be blunted with the time span of stress concentration from 1 ms to 600 s. It is also shown that the blunting of hydraulic fracture is a highly localized process. The morphology of the blunted crack depends on the stress distribution in the vicinity of the crack tip.