Menstrual cycle is ideally regular in every months in the length of time about 21-35 days at each menstrual period. Menstruation on women is regular after 18 years old. Result of the research on 4000 women is that there are only 3% of who experience regular menstrual cycle. Almost all women experience irregular menstrual cycle from a month to another. Many factors affect irregular menstrual cycle, one of them is stress. The aim of the research is to find out relation of stress and menstrual cycle on inclusive line students of 18-21 years old at 3rd Diploma of Obstetrics Course of Study in Balikpapan Health Polytechnic, Ministry of Health East Borneo. The research is characterized by analytic descriptive with cross sectional research design. Population of the research are obstetrics students of 18-21 years old with 197 number of students. Sampling is by stratified random sampling method with 132 persons. Data collection method is using questionnaire as data collection instrument. Data analysis technic is by using chi square (X2) statistics test with CI =95% and α = 0,05. Statistics calculation is using computer program. Result of the research shows that there is significant relation between stress and menstrual cycle with the value of Pvalue = 0,001 < the value of α = 0,05 with X2 calculation = 14,915 > X2 table= 3,841. Conclusion: most of inclusive line students of 18-21 years old at 3rd Diploma Obstetrics Course of Study in Balikpapan are stressful.