The present study was a randomized, controlled trial examining the effect of zinc sulphate in preventing of mucositis in patients with leukemia under chemotherapy.The inclusion criteria of the study were leukemia patients with 18 years old or above, under chemotherapy, be alerted and oriented, and able to swallow capsule and did not show the mucositis symptoms such as ulcers, erythema, oedema, painful and dysphasia.The patients with any ulcer in their mouth, pregnant and lactating women, individuals under head and neck radiotherapy and patients who had the allergic to zinc were excluded from the study. The participants were allocated into the intervention (zinc sulphate) or the control group (placebo capsules) by block randomization with a random sequence of 2 or 4 block sizes. In this study, the patients remained blind to their allocation into the intervention and control groups. The researcher assistant who collected the data was also blind to the study groups. Moreover, the person who performed the data analysis was kept blinded to the allocation. The sample size was 86 patients (43 in each group). Both groups received either zinc sulfate (50 mg) or placebo capsules three times daily for 14 days from the first day of chemotherapy. The mucositis was measured by JP Donnelly and Colleagues questionnaire and World Health Organization mucositis scale in fourth, seventh and 14th day of starting chemotherapy.