The lifestyle of the Adolescents is a major issue in the today's society and it differs though nature & experience of individual, so here is the study was conducted to assess the effectiveness of planned school health awareness programme on the knowledge and practice of adolescents about healthy lifestyle in selected school of Dehradun. A quantitative research with quasi-experimental approach & one group pre-test post-test design was adopted. Convenient sampling technique was used. Fifty subjects were included in the study. The analysis of pre-test and post-test knowledge and practice score using paired t-test revealed that there was significant improvement. The mean of the post-test knowledge and practice score (11.72 ± 3.26) and (27.38 ± 3.48) was found to be higher than the mean of the pre-test knowledge and practice score (7.06 ± 2.17) and (23.7 ± 3.95). There was association between found in pre-test and post-test knowledge and practice score. The study concluded that the planned school health awareness programme resulted in significant improvement of knowledge and practice about healthy lifestyle among adolescents which was very helpful healthy individuals and their family, thus society in future. Adolescence, is the second decade of life, is a period in which an individual undergoes major physical and psychological changes. It presents a window of opportunity to set the stage for healthy and productive adulthood and to reduce the likelihood of problems in the years that lie ahead. At the same time, it is a period of risk: a period when health problems that have serious immediate consequences can occur or when behavior problems that could have serious adverse effects on health in the future are initiated. 2 According to Indian censes data more than 50% population area covered by adolescents. Adolescent Health is the optimal state of wellbeing in all areas of life-physical, emotional, cognitive, social and spiritual. By using the broad definition of health adolescents are healthy when they engage in healthy behavior that contribute to a healthy lifestyle, they have the capacity to thrive in spite of stressors in life and they successfully engage in the developmental tasks of adolescence; and experience a sense of wholeness and well-being. 3