This research work, conducted during 2016-2017, was aimed at understanding primarily about sexual behaviour and its determinants among the adolescent girls aged 10-19 years residing in Tamil Nadu state.Examinations of the prevalence of pre-marital sexual behavior, and their background characteristics closely link, thus, the results indicated that a substantial percentage of adolescent girls are indulging in pre-marital sex irrespective of their background characteristics. A large majority of the adolescent girls surveyed had experienced in love (66.2 %), most of the girls' proposed by the boys as the same girls also propose her love with boys. Premarital experience is more in age of 16-17 years and slightly more in 14-15 years. Most of the girls' sexual experiences were mutually, happened at respondents/first partner house, enjoying pre-marital sex like hugging, kissing and sexual intercourse and never used any contraceptive method. The major determinants of sexual behaivours were income, occupation, education and family, society and workstation situation. Consequences was excessive vaginal discharge, sour smelling stirringly discharge, and somatic complaints, anxious/ depression.