Background: Second generation antipsychotics (SGAs) are strongly associated with accelerated body weight gain resulting in overweight and obesity in patients with severe mental illness (SMI). Evidence found that lifestyle interventions used behavioral techniques to improve dietary habits and increase physical activity can reduce the increase in patients’ body weight and BMI. Methods: Randomized Controlled Trial (RCT) on 54 patients with severe mental illness were randomized with 27 patients joined in the Lifestyle Intervention group (LIP) and 27 patients in treatment as usual group (TAU). The intervention group attended the educational talks on medication, diet control and physical exercise and use of a daily record. Result: After twelve months, the LIP presented a decrease of 1.25kg (CI 95% -0.73 to 3.24) and treatment as usual TAU presented with a significant increase of 1.65kg CI 95% -2.84 to -0.45) (p=0.009). The BMI of the LIP showed a decrease of 0.51kg/m2 (CI 95% -0.29 to 1.33) and the TAU presented an increase of 0.66kg/m2. Conclusion: The TAU shows a significant increase of body weight while the LIP group had a decrease in body weight under the Life Intervention program. This study was not intended for patient to reduce body weight, rather to prevent their body weight to increase and enhance their quality of life through understanding physical health, exercise and dietary habit.