This study examined the perception of Lagos state secondary school students on influence and effectiveness of sex education on sexuality behaviour. Theoretically, the study is grounded in the Perception theory with focus on selective process theory and Uses and Gratification Theory. Descriptive survey method was adopted and data was collected using self-administered, structured close-ended questionnaire. 480 respondents were randomly selected from eight secondary schools from four local governments in Lagos State, Nigeria. Data collected were analysed using Pearson Product-moment correlation. Results indicated that there is a significant positive relationship between sex education and sexuality behaviour of secondary school students (r=0.34), there is a significant positive relationship between sex education and the effectiveness of sex education to sexual behaviour (r=0.24), and also there is a significant positive relationship between effectiveness of sex education and sexuality behavior of secondary school students. (r=0.39).nbsp The results also showed that majority of the students are aware of sex education which has different meaning to them. 63.8% of the students have received sex education with 53.6% of the students indicating from their teachers. 83.9% of the respondents agreed that sex education is relevant in stimulating their interest in sexual behaviour and 75.0% agreed that sex education is relevant in holding their interest in sexual development. The study recommended that parents/guardians should show more interest in the sexual development of their children/wards. Schools should organise effective communication programmes and seminars about sex education for the secondary school students.