Objective: To explore heterosexual and homosexual behaviors, and influencing factors, in those unmarried out-of-school young people aged 15 to 24 years, so as to provide the basis on the education and service of sexual and reproductive health for those young people. Methods: The questionnaire survey form was anonymously filled out by 1 113 unmarried out-of-school young people on six target spots in urban and rural areas of six provinces by the computer assisted self-interview (CASI) system. Results: The incidences of hetero-or homo- sexual behaviors in the 15 to 18 years male group and the 19 to 24 years male group were significantly higher than those in female groups (P < 0.05). 7.14% of the first heterosexual behaviors in the 15 to 18 years female group were forced. Sexual safety precautions were used in 66.67% of males and 40.00% of females in the 15 to 18 years group, and in 58.82% of males and 49.11% of females in the 19 to 24 years group. About 30.00% of males reported that they had partners' pregnancies, and 30% of females reported that they had pregnancies. The incidence of multiple sexual partners in the 19 to 24 years male group was significantly higher than that in the age-matched female group (P < 0.01). On homosexual behaviors, the average age of the first homosexual behaviors in the 15 to 18 years group was about 14 years, 15.79% of males in the 19 to 24 years group were forced to have, and 46.15% of males and 33.33% females reported that they had more than two sexual partners. Multivariate analyses showed that watching pornographic materials and being older were common risk factors of heterosexual behaviors for males (OR, 3.51 and 3.81 respectively) and females (OR, 6.41 and 3.85 respectively). Compared with those factors approving premarital sexual behaviors, the attitudes rejecting premarital sexual behaviors from respondents themselves (OR = 0.18) and their parents (OR = 0.37), and the neutral attitude from friends (OR =0.47), were protective factors in males, while the options that the female respondents thought it being not good but acceptable (OR =0.14) or opposed (OR = 0.06), and that friends thought it being unacceptable (OR = 0.16) or unknown (OR = 0.21), were protective factors and the neutral attitude from parents was risk factor (OR = 12.56) in females. Conclusions: The incidences of heterosexual and homosexual behaviors, and unsafe sexual behaviors, in those unmarried out-of-school young people were high, which were positively related to the pornographic material and the open attitudes from respondents themselves, parents and friends. [ABSTRACT FROM AUTHOR]