Background: The prevalence of obesity and overweight among adolescents is increasing in developing countries day by day owing much to the sedentary lifestyle habits like lack of physical activity and increase in intake of junk foods. Obesity is has become a serious public health problem and it is a definite precursor for diseases like CVD, Diabetes etc. Objective: To find out the prevalence of Obesity among the school going adolescents and to assess the various socio–demographic, physical activity and dietary factors related to Obesity. Materials and Methods: This is a school based cross–sectional study carried out in the Government Higher Secondary schools of Guwahati city, Assam. A multistage sampling technique was applied to achieve a sample size of 528 students for the study. A predesigned, pretested semi–structured schedule was used to collect necessary information. Results: The overall prevalence of Obesity was found to be 3.41%. The prevalence of obesity in Boys is 3.03% and in Girls it is 3.79%. It is found that obesity is significantly associated with physical activity, intake of fast food and soft drinks. Conclusion: The high prevalence of obesity and overweight among the students demands an urgent need for awareness raising campaigns to increase their knowledge about health hazards of obesity.