The purpose of this study is to investigate the effects of sex education program which helped middle school students to acquire correct knowledge and positive attitude toward sex. This research was designed to do a quasi-experiment of pre-test & post-test with a nonequivalent control group. The subject of study were 82 males who are in the second grade in middle school student: Forty-one students were experimental group and the same of them control group in Pusan. The subjects were pre-tested and post-tested with Chie Hae Young sex knowledge test & sex attitude test translated by researcher. The subjects in the experimental group participated on 13sessions (total 25 hours) of sex education program which was developed by researcher and each session lasted for 120 minutes. The subjects in control group did not receive any treatment. The statistical analysis contains real number, percentage, average, standard deviation, X^2 -test and t-test from SPSS. The results of this study were as follows. 1. Regarding to the first hypothesis it was supported as the following: `The scores of sexual knowledge in the experimental group that was given sex education will be higher than those of the control group` (t= 9.627, p=0.000). 2. Regarding to the second hypothesis it was supported as the following: `The scores of sexual attitude toward sex in the experimental group that was given sex education will be higher than those of the control group` (t=6.595, p=0.000). From the test of Sex Education Program, the following conclusions were obtained. First, The sex education program was effective for the subjects in helping to obtain correct knowledge and positive attitude toward sex. Second, this sex program proved to be adaptable for various who desired students and they were very satisfied with it. The result of the study were suggested it is so necessary that school nurses should be performed sex education programs regularly at school.