This study aimed to evaluate the effectiveness of Reality Therapy Theory on self- harm behaviors and functions in high school students in Shiraz, Iran. A semi-experimental methodology with a pretest-posttest design and a control group was used. The statistical population included female high school first-year students from (S) high school in Shiraz in the academic year of 2017-2018. The purposeful sampling method was used in this study where 30 students were randomly selected from among all high school first-year students studying in the specified school whose scores in the Self- Harm Behavior Questionnaire (SHBQ) ranged within the first three quarters. Participants were randomly assigned either to an experimental or a control group (15 participants in each group). The experimental group received eight minute training sessions based on the concepts of reality therapy. Klonsky & Glenn’s (2011) self- harm questionnaire was used to collect data. At the level of descriptive statistics, the mean and standard deviation were used. Data were analyzed using analysis of covariance (ANCOVA). The findings showed that reality therapy training reduced the students’ self-harm functions; and had a significant positive effect on the extra-personal and intrapersonal functions of the students' self- harm behaviors تفاصيل المقالة