Our previous study shows that short term high-intensity interval training (HIT) under normobaric hypoxia simulated at 2500 m has an additive effect on the improvement of cardiorespiratory fitness in overweight young women. The purpose of this study was to investigate whether further improvement can be observed in aerobic and aerobic capacity as well as blood lipids if hypoxic stress were maintained throughout the HIT intervention period. Methods: A single-blind randomized controlled experimental design was applied. 42 sedentary young women (age: 22 ± 2 years, BMI: 23 ± 3 kg·m-2) completed the HIT under normoxia (NORM, n = 14, FIO2: 0.21), or normobaric hypoxia simulated a stable altitude of 2500 m (HYPS, n = 14, FIO2: 0.15) or a graded altitude of 2500-3400 m (HYPG, n = 14, FIO2: 0.15-0.14) for 4 weeks. HIIT was composed of 80 repetitions of 6 s maximal cycling effort interspersed with 9-s recovery per day, for 3 days per week. Cardiorespiratory fitness [peak oxygen uptake (V̇O2peak), anaerobic capacity with two Wingate-based test and serum lipid profile (triglycerides, total cholesterol, high-density lipoprotein cholesterol, and low-density lipoprotein cholesterol ) were assessed within 4 to 5 days at pre- and post-intervention. Habitual physical activity and diary behavior were maintained during the intervention period. Results: With similar daily energy intake and physical activity, four weeks HIT resulted in significantly improvements in V̇O2peak [NORM: 0.7 ± 2.7 ml·kg-1·min-1 (+3.4%), HYPS: 2.6 ± 2.8 ml·kg-1·min-1 (+10.2%), HYPS: 3.8 ± 2.8 ml·kg1·min-1 (+15.1%)]. Compared to NORM, the increments of V̇O2peak in HYP (HYPS: p > 0.05, d = 0.63, HYPG: p < 0.05, d =1.02) were greater. Moreover, no group effect was found in anaerobic capacity despite significant improvements in peak power, average power, relative average power and power decline rate (p < 0.01). There were no significantly changes on the serum variables after the intervention. Conclusion: 4-week of HIT improved cardiorespiratory fitness and anaerobic capacity in sedentary young females. Furthermore, it seems that the additive effect of the HIT under normobaric hypoxia is more effective when hypoxic stress can be maintained.