Background:Smokers in indonesia was still high especially teenagers. Due to smoking for health the cancer , heart trouble , a disorder of pregnancy and stroke. To prevent the smokers in adolescents needs to be know level of teenagers Purpose: To determine the knowledge of adolescent in SMP 2 Sanden, Bantul, Yogyakarta Method: This is a descriptive research method and the sample with drawal is done by using Simple Random Sampling technique witha sample size of 273 people. An instrument for the data is a questionnaire covered withmultiple choice answers Result: The Study knew that the level adolescent of knowledge there was good 15 (17.5%), enough 31 (36%) and less 40 (46.5%) Conclusion and advice: Research the knowledge junior high school students 2 sanden about the dangers of cigarettes less. Advice to nurses should to do health counseling to schools