Background of the Study: Adolescence is a very critical and important phase in one’s life. It is a stage with unique biological and social characteristics of its own. Most of the physiological, psychological, and social changes occur during this period. About one fifth (22.5%) of the total population in India is between the age group of 10-19 years. Our modern eating environment has an effect on the way adolescent eat. Junk food consumption tends to the occurrence of many life-threatening diseases during adulthood and later life. Studies proved that educational programme might be effective in influencing adolescents to choose a healthier diet. Hence the investigator felt the need to provide planned teaching programme on health hazards of junk food among adolescents in a selected Pre- University College at Trivandrum. Objectives: The objectives of the study were to- 1. Determine the existing level of knowledge on health hazards of junk food among adolescents in a selected Pre-University College by using structured knowledge questionnaire. 2. Evaluate the effectiveness of planned teaching programme on health hazards of junk food among adolescents in a selected Pre-University College by using same structured knowledge questionnaire. 3. Find the association between pre-test knowledge score on health hazards of junk food among adolescents with selected demographic variables. Methods: The research design was one group pre-test post-test design and the study was conducted at a Pre-University College, Trivandrum. The sample consisted of 110 adolescents who were selected by simple random sampling technique. Data collection was done by using the demographic variables, structured knowledge questionnaire for assessing the knowledge of adolescents regarding health hazards of junk food. After assessing the knowledge of adolescents a  planned teaching programme was given on the same day. A post-test was conducted on the 7th day after the PTP to find out the gain in knowledge among adolescents regarding health hazards of junk food. Results: Majority of the subjects 85(77.27%) had moderately adequate knowledge score in the post-test, whereas in the pre-test none of the samples had adequate knowledge. In the post-test none of the sample had inadequate knowledge whereas in the pre-test 28(25.45%) samples had inadequate knowledge. The mean post-test knowledge score (19.32) was higher than mean pre-test knowledge score (13.18) suggesting that PTP helped in improving the knowledge of adolescents regarding health hazards of junk food. The mean difference between the post-test and pre test knowledge scores of adolescents regarding health hazards of junk food was found to be highly significant at 0.05 level. There was no significant association between mean pre-test knowledge score and selected demographic variable at 0.05 level of significance.