Purposes of this research were to study and compare the effects of core body training on exercise ball and floor exercise upon back muscle strength and flexibility. The subjects were 30 rhythmic gymnast at the age of 9-15 years old from Jintanagymnastics Club. They were divided into 3 groups of 10 by random assignment. The control group performed their usual rhythmic gymnastics, the experimental group 1 performed their usual rhythmic gymnastics in combination with the core body training on exercise ball, and the experimental group 2 performed their usual rhythmic gymnastics along with the core body training on floor exercise. The experimental groups participated in the training programs 3 days a week for 8 weeks. Data were analyzed using mean, standard deviation, one-way anova with repeated measures, one-way analysis of covariance and multiple comparison testing by Tukey’s method at the .05 level of significance. The results indicated that the back muscle strength after 8 weeks of two experimental groups were significantly different from the control group (p< 0.05). There were not significant differences in back muscle strength after 8 weeks between the experimental group 1 and the experimental group 2. In addition, the results showed that the increased percentage of back muscle strength and flexibility of experimental group 1 using exercise ball were better than that of the experimental group 2 using floor exercise and that of the controlled group.