Reading is the basic literacy as the foundation for intellectual development. Researchers found that Indonesian students have low reading competence. This is due to the fact that they do not possess a good reading habit. Hence, the study aimed at investigating the effect of local culture-based storybooks, one of which is a fable on students' reading competence and analyzing the effect size of the treatment. It was an experimental research administering post-test only control group design. The population was sixth grade students of elementary schools in Buleleng Regency, Bali. The samples were determined using intact group cluster random sampling technique of 34 students from two elementary schools. Experimental group was treated by using storybook, whereas control group was taught by using text-based story. The data were collected through post-test. The obtained data were analyzed descriptively and inferentially using t-test. Cohen's d was administered to check the effect size. The result proves that there was a significant effect of local culture-based storybook on students' reading competence, while Cohen's d shows that the effect size was medium. Thus, teachers are recommended to regularly use stories for improving reading competence.