(i) The aim of this study was to evaluate the effect of acute hypoxia and exercise intensity on physiological parameters during equivalent relative intensity exercise. (ii) Eight healthy male subjects [mean (SD) age 25 (4) years, height 171.0 (4.5) cm, body weight 65.3 (6.9) kg] participated in this study. Subjects performed the incremental exercise tests in both normoxia and acute normobaric hypoxia (FiO₂ = 14.4%). Thereafter, subjects received four sessions of the constant- load exercises, which consisted of exercises at 40% and 60% maximal oxygen uptake (VO_2max) in the respective conditions. Respiratory parameters, heart rate (HR), blood lactate (BLA) were taken during the incremental exercise tests. We assessed HR, BLA, and rate of perceived exertion (RPE) during the constant-load exercises. (iii) At 40% and 60%VO_2max during the incremental exercise tests, HR and BLA were not significantly different between hypoxia and normoxia. Also, HR, BLA, and RPE were not significantly different between hypoxia and normoxia during the constant-load exercise tests at 40%VO_2max. However, BLA was significantly higher during the constant-load exercise at 60%VO_2max in hypoxia, even though HR and RPE were not significantly different compare to normoxia. (iv) During the constant-load exercise tests, the HR-BLA relationship was remained at the low exercise intensity, nevertheless, BLA was higher in hypoxia at the moderate exercise intensity. Our result suggests that the HR-BLA relationship could be changed during the constant-load exercise at equivalent relative intensity in acute hypoxia, which depends on the exercise intensity. Furthermore, this phenomenon could not be assumed by the preliminarily performed the incremental exercise test.