Background: food habits and lifestyle significantly affect the prevalence of overweight and obesity thus impact the body mass index (BMI) among Saudi population. Objectives: assessments of the impact of lifestyle on the BMI among the adult population in Kingdom of Saudi Arabia (KSA). Subjects and Methods: a cross sectional study conducted on 1055 adult Saudi subjects aged between 20-40 years old. A predesigned questionnaire was distributed among included subjects from January 2017- May 2017consisted of questions about subject's demographics, anthropometric measures, and lifestyle. Results: this study included 1055 subjects from which 40.9% normal, 5.9% were underweight, 25% were overweight, 17.1% were obese, 8.4% were severely obese and 2.7% were morbidly obese. Being female, older age and smoking were significantly related to overweight and obesity. The bad lifestyle habits, lack of exercise and watching TV and video games were significantly associated with a high prevalence of obesity and overweight among studied subjects. Conclusion: bad lifestyle habits, smoking, not practicing physical exercise and using entertainment means for a long time significantly increased the BMI thus revealing high rates of overweight and obesity.