Abstract: Background:_. Children find themselves amidst of a complex society that is undergoing breathtaking changes. Concepts, relationships, lifestyles are metamorphasised to accommodate the new jet-setting age. Food is of no exception, Healthy nutritious foods have been replaced by the new food mantra – JUNK FOOD! Junk food comprises of anything that is quick, tasty, convenient and fashionable. It seems to have engulfed every age, every race and the newest entrants in children. Objective:1.To assess the knowledge regarding harmful effects of junk food among high school students.2.To find the association between the knowledge regarding harmful effects of junk food with their selected demographic variables among high school students. Material and Methods: Quantitative approach and non-experimental descriptive research design. The study was conducted in Rm.A.S.Packirisamy pillai High School, Karaikalmedu, Karaikal.60 samples of High school students were selected by non-probability convenient sampling technique. The tool consists of two sections. Section-A consists of semi structured interview schedule to elicit the demographic variables and Section-B consists of semi structured interview schedule regarding harmful effects of Junk food. Result: The study result shows that majority 83.3% (50) of High school students had inadequate knowledge, 13.3% (8) had moderate knowledge, and 3.3% (2) of them had adequate knowledge regarding harmful effects of junk food. There is a significant association between the knowledge on harmful effects of fast food with their demographic variables such as age, class of studying, education of the mother and source of information regarding junk food. There is no association with respect to other demographic variables such as religion, gender, type of family, monthly income, education of the father, occupation of the father, occupation of the mother, previous knowledge on effects of junk food on health.