Purpose: This study has examined effectiveness of a secondhand smoking prevention program for adolescents. Methods: The study was done in a nonequivalent control group pretest/posttest design. The subjects of the current study were 198 middle school students from K city in the Gyeongbuk province. The students were assigned to an experimental group (100) and participated in the secondhand smoking prevention program or to a control group (98) and did not take part. Data collection was done from June to July 2012. A Chi-square test, independent t-test, paired t test and ANCOVA were used with the SPSS 18.0 program for data analysis. Results: There were significant differences between the groups in knowledge of short-term influence of secondhand smoking (F=9.65, p<.005), knowledge of long-term influence of secondhand smoking (F=15.53, p<.001), verbal coping skills for secondhand smoking prevention (F=16.35, p<.001), behavioral coping skills for secondhand smoking prevention (F=8.49, p<.005), and assertiveness of secondhand smoking prevention (F=17.30, p<.001) measurements. Conclusion: The secondhand smoking prevention program delivered to the adolescents is an effective method of encouraging secondhand smoking prevention and can be utilized as an effective nursing intervention for adolescents.