Despite several campaigns that have been conducted for HIV/AIDS reduction and the knowledge about HIV/AIDS people have from campaigns and seminars, the rate of HIV infection is still high due to risky sexual behaviour among sexually active adults who include secondary school students. Therefore, the broad objective of this study was to determine the level of awareness of HIV/AIDS and sexual behaviour among secondary school students in Tanzania. The specific objectives were to assess the level of awareness about HIV/AIDS among secondary school students, determ- ine the sexual behaviour among secondary school students, determine the correlation between awareness about HIV/AIDS and sexual behaviour, and identify barriers to behavioural change. Data for this study were collected in Njombe District through structured interviews using a questionnaire, focus group discussions, and key in- formant interviews. A total of 240 secondary school students from form one to form three in four selected schools were interviewed in October and November 2010. The study revealed that the awareness of HIV/AIDS transmission and prevention among secondary school students was high, and the mean number of points scored on awareness of HIV/AIDS was 82.3 out of 100 on an index scale. The study findings on sexual behaviour showed that 69 (32.9%) of all 240 respondents had worse sexu- al behaviour, based on an index scale of sexual behaviour which had a maximum of 110 points, and 71 to 110 denoted bad sexual behaviour while less than 71 denoted good sexual behaviour. The results indicated that there was positive correlation between awareness about HIV/AIDS and sexual behaviour (r = +0.050) but the cor- relation was not significant (p = 0.680). Findings on barriers to behavioural changes showed that factors that were perceived to contribute to students’ involvement insexual intercourse were sexual desire, parents/guardians giving them too little money, lack of knowledge on HIV/AIDS, to get money, and persuasion by peers. Based on the correlation results which showed that there was no significant correla- tion between awareness of HIV/AIDS and sexual behaviour, it is concluded that awareness of HIV is not the only determinant of sexual behavioural change, and that secondary school students with lower and higher awareness of HIV/AIDS are likely to have the same sexual behaviour. In view of that conclusion, it is recommended that education on HIV/AIDS should be given continuously to secondary school stu- dents so that they can always remember the risk of HIV infection and hence avoid bad sexual behaviours.