B BACKGROUND :Body mass index is a simple measuring instrument for monitoring nutritional statusrelated to underweight and overweight. Increased BMI correlates with the activityof autonomic nervous system which can reduce the activity of parasympatheticnervous system and increase the activity of sympathetic nervous system. Bothautonomic nervous systems work in opposite ways nonetheless they have to bebalanced, the imbalance between sympathetic and the parasympathetic nervoussystem causes a decrease in the HRV value. Therefore, a heart rate variabilityparameter is needed to detect the balance of autonomic activity when there is achange in body mass index. The purpose of the research is to determine thecorrelation between body mass index and heart rate variability in young adults.METHODS :This study was carried out with the cross sectional method. The sample amountedto 75 respondents obtained by consecutive non-random sampling. The study wasconducted on February 2020. Body mass index assessment is done by using a scaleand a stature meter. Assessment of heart rate variability is done by using thewelltory application on smartphones with photoplethysmography technology.Research data is analyzed with pearson correlation test using SPSS for windowsversion 25 with a significance value of p < 0,05.THE RESULTS :Pearson correlation test shows that body mass index is a factor which affects theautonomic nervous system while autonomic activities can be analyzed with hrvparameters, the higher the body mass index, the lower the heart rate variability (r=-0,458; p= 0,000).CONCLUSION :There is negative correlation middle degree with significance value between bodymass index with heart rate variability in young adult.