The critical power test provides 2 measures, critical power (CP) and anaerobic working capacity (AWC). In theory, the CP measurement represents the maximal power output that can be maintained without fatigue, and AWC is an estimate of work capacity associated with muscle energy reserves. High-intensity interval training (HIIT) has been shown to be an effective training method for improving endurance performance, including VO2PEAK. In addition, creatine (Cr) supplementation has been reported to improve AWC without training; however, it has shown no effect on CP. The purpose of this study was to examine the effects of 4 weeks of HIIT with Cr supplementation on CP and AWC. Forty-two recreationally active men volunteered to participate in this study. Participants were randomly assigned to 1 of 3 groups: Cr (n = 16), 10 g Cr + 10 g dextrose; placebo (PL, n = 16), 20 g dextrose; control (CON, n = 10), no treatment. Before and after supplementation, each participant performed a maximal oxygen consumption test VO2PEAK on a cycle ergometer to establish peak power output (PPO). Participants then completed a CP test involving 3 exercise bouts with the workloads set as a percentage of their PPO to determine CP and AWC. After a 2-week familiarization period of training and supplementing, PPO, CP, and AWC were remeasured before an additional 4 weeks of HIIT and supplementation were completed. Training consisted of 5 sets of 2-minute exercise bouts with 1 minute rest in between performed on the cycle ergometer, with intensities based on PPO. A significant improvement in CP was observed in the Cr group (6.72% +/- 2.54%), whereas PL showed no significant change (3.87% +/- 2.30%), and CON significantly decreased (6.27% +/- 2.38%). Furthermore, no changes in AWC were observed in any of the groups after treatment. The current findings suggest that Cr supplementation may enhance the effects of intense interval endurance training on endurance performance changes.