Introduction: Cancer can cause emotional stress in parents, which has a negative impact on the quality of their life. Also, anxiety and psychological stress have a negative effect on the health of parents, and fatigue causes a sense of weakness and reduces the capacity for mental and physical activity, and insomnia, as well as stress and inability to perform their occupational and social functions. This study aimed to determine the effect of relaxation techniques on anxiety, fatigue, and sleep quality of parents of children with leukemia under chemotherapy in South East Iran in 2015. Methods: This is a randomized controlled trial study. The study population included parents of children with leukemia undergoing chemotherapy who were admitted to a teaching hospital in South East Iran. One hundred twenty parents were randomly assigned to control and intervention groups, and the experimental group was provided with Benson relaxation technique. Data collection tool included a demographic questionnaire, state-trait anxiety inventory, Brief Fatigue Inventory, and sleep quality inventory. Data analysis was done by SPSS 16 and paired t-test, Wilcoxon, Mann-Whitney, regression, One-Way ANOVA and Pearson tests were performed, and p ≤ 0.05 was statistically significant. Results: The mean score of state anxiety in the intervention group was 60.86 ± 8.95 and 35.95 ± 4.61 before and after the intervention, respectively. The mean score of trait anxiety was 56.56 ± 4.75 and 34.45 ± 4.95. The mean score of the fatigue was 73.83 ± 14.63 and 43.71 ± 11. 06, and the mean score of the quality of sleep was 13.5 ± 6.05 and 5.7 ± 3.43 before and after the intervention respectively. There was a statistically significant difference among state-trait anxiety, fatigue, and sleep quality in intervention and control groups after the intervention. There was a statistically significant negative correlation between fatigue and age, but there was no statistically significant relationship among the mean fatigue, weight, the number of sons and daughters, education, occupation, gender, place of residence and income (p> 0.05). There was no statistically significant relationship among the quality of sleep of parents, education, gender, and place of residence, but there was a statistically significant relationship between state anxiety and education (p≤0.05). Conclusion: The results can predispose family-centered nursing care to support more the parents of children with cancer in the face of the stress of illness. Developing programs for training muscle relaxation techniques will improve family functioning and mental health.