The purpose of this study was to examine effects of sling exercise with trunk stability technique on trunk muscle thickness and flexibility in adolescent with low back pain (LBP). Thirty subjects participated in the current study, adolescents with chronic LBP were selected randomly divided to 2 groups: general sling-trunk stability exercise (GSE) group (n=15) and sling-trunk stability exercise using PNF technique (SEP) group (n=15). These subjects received individual sessions program 5 times a week for 4 weeks, GSE group trained general bridge exercise with sling, SEP group trained rhythmic stabilization exercise of PNF technique with sling. The trunk muscle thickness and flexibility were measured before and end of the exercise program. The trunk muscle thickness was significantly improved in the SEP group (p<.005) and the SEP group showed significantly different form GSE group at post of exercise (p<.005). The trunk muscle flexibility was significantly increased in the two groups (p<.005) and the SEP group showed significantly different from GSE group at post of exercise (p<.005). The results suggest that sling-trunk stability exercise using PNF technique may be appropriate for improving trunk muscle thickness and flexibility in adolescent with LBP.