Physical exercise plays a crucial role in maintaining the overall health and well-being of individuals, especially during adolescence, which is a critical period for physical development. The main purpose of this study was to examine the comparative effect of eight-week HIIT vs. MICT on some selected physiological variables of healthy untrained Male adolescents. Agewmidir higher preparatory school was selected as the study area and by using the G-power sample size determining technique, 40 adolescents aged 15-18 years were selected and grouped randomly into HIIT (N=20), age:16.55 ± 1.146 and the MICT (N=20), age: 16.10±0.912 intervention groups. True experimental study design specifically pretest-posttest randomized two-group design was used. Both group participants had taken consecutive eight-week HIIT and MICT interventions for 3 days per week. Participants' pre-post quantitative data was taken primarily with Omron blood pressure monitor, Queens college 3-minute step test, OMRON HBF-514 body composition analyzer, and meter were used with consideration of data collection procedures and protocols by the researcher with the help of a professional assistant in this study. SPSS26 version software with descriptive and ANCOVA was used to analyze and interpret the findings of comparative results. from the result of the findings of study variables both intervention groups have a significant effect with p value less than 0.05 on HR (p = 0.004), SBP (p = 0.003), DBP (P = 0.007), VO2max (P = 0.000), and BF% (p = 0.00) with significant mean difference having HIIT better improvement on HR, SBP, VO2max BF% of adolescents when pretests of all variables are controlled as covariate. The mean difference in DBP reduction of participants was higher in the MICT groups rather than the HIIT group when the pretest DBP was controlled as a covariate. However, both intervention groups have no statistically significant mean difference with a p-value greater than 0.05 on the BMI (p = 0.092), and the waist-to-hip ratio (p = 0.133) of participants. In summary, HIIT intervention on adolescents' key physiological variables has a significantly great improvement in average compared to the MICT group, as a result, adolescents are recommended to do HIIT intervention with its time efficient and enjoyable nature.