Abstract: The study investigated the influence of family structure on the social behaviour of adolescents. The research design used in this study was survey. A sample of 398 adolescent students were selected through stratified random sampling technique from 161 public Senior Secondary Schools in Ekiti State. The population for the study was made up of all the adolescent students in secondary schools in Ekiti State. The sample for the study consisted of (398) adolescents selected from four secondary schools in the three senatorial districts of Ekiti State. The selection was achieved through multi-stage Sampling technique. The face validity procedure of the instrument was determined through critical examination and evaluation of the items on the questionnaire by experts in Guidance and Counselling and also in Tests and Measurement. A test re-test method was used to ensure the reliability of the instrument. The reliability coefficient of the instrument was estimated at 0.97. The instrument for collecting data was a self-designed questionnaire titled; Family Structure Questionnaire (FSQ) containing 40 items. Descriptive statistical analysis was done using frequency counts, percentages, standard deviation and means while t-test statistics and Pearson Product Moment Correlation (PPMC) were used in the data analysis, 4 research questions were raised to guide the study and 4 corresponding hypotheses were formulated and tested at 0.05 level of significance. The results of the analysis showed that the family structure of the adolescents influence their social behaviour. It was found out that adolescents from single parent homes and those from both parents differ significantly in their social behaviour. Based on the results, it was recommended that the father and the mother should stay together for good upbrining of their children. Also that couples should try to prevent separation in their marriage, so that their children will not be influenced negatively.