Abstract：Purpose: This study was aimed to discern the correlations among sex, cardiopulmonary function and heart rate variability in young adults. Methods: 120 college students were recruited in this study. Each subject was asked to arrive to laboratory between 9:00-11:00 a.m., to complete two measurements including heart rate variability analysis by wrist monitor of heart rate and cardio-respiratory function by 3-minute step test. The data were analyzed by t-test and Pearson product-moment correlation coefficient. Results: The LF% and LF/HF of males were higher than that of females, while the HF% of males was significantly lower than that of females. Cardio-respiratory endurance evaluated by 3-min step test was positively correlated with the SDNN, pNN50, RMSSD, TP, HF, and LF. The pNN50 is most related to the 3-min step test score in respect to other HRV indexes. Conclusion: The activities of sympathetic nervous system in males were higher than those in females, and the activities of parasympathetic nervous system in males were lower than those in females. Individuals with better cardio-respiratory function might have higher HRV and better modulation of autonomic function. (Full text)