Objective : The purpose of this study was to identify the effect and difference of abdominal muscle strengthening exercise, back muscle stretching and multi-training on the lumbar flexibility of 20s adults. Method : The abdominal muscle strengthening exercise, back muscle stretching and multi-training were conducted 9 times targeting 21 subjects who attended K University from 2013 May 29 to June 14. Sit and Reach Tests were conducted 2 times(before and after exercise program) for flexibility test and measured data were processed with SPSS program WIN 12.0K. By the Wilcoxon signed rank test, the effectiveness of exercises are verified. By the Kruskal-Wallis test and Mann-Whitney test, the difference of effectiveness among the exercise groups are verified. Result : The results of this study were summerized below 1. Before and after exercise, in abdominal muscle strengthening exercise group, back muscle stretching group and multi-training group are showed statistically significant differences(p<.05), 2. There were statistically significant difference in the improvement of the flexibility between each group(p>.05). Conclusion : These data suggests that all of the 3 exercise programs are brought positive influences on the improvement of flexibility, and abdominal muscle strengthening exercises and flexibility multi-training are effective on the flexibility more than, back muscle stretching in 20s adults.