Background: Magnesium oxide is proposed to be effective in prevention of nephrotoxic impact of carboplatin therapy. The aim was to evaluate the magnesium oxide supplementation effect on serum creatinine, blood urea nitrogen (BUN) and glomerular filtration rate (GFR) in cancerous children. Methods: 18 children were treated with 250 mg/day magnesium oxide and other 18 children with placebo for 14 days before onset of carboplatin chemotherapy based on the routine protocols in oncology. Serum creatinine (Cr), BUN and GFR were measured and compared before, on day 3 and 7 of magnesium carboplatin therapy. Results: There were 7 (38.89%) and 12 (66.67%) male patients in treatment and control groups, respectively. Serum Cr and BUN were increased significantly 3 and 7 days after intervention in both groups. There was an insignificant difference for serum Cr and BUN between treatment and control groups before intervention, 3 days or 7 days after carboplatin administration onset (p> 0.05). The GFR was reduced significantly from 101.38±14.67 to 90.11±10.52 mL/min/1.73m² in treatment and from 97.5±9.71 to 92.33±10.61 mL/min/1.73m² in control group. After 7 days the GFR was reduced to 84.11±12.47 mL/min/1.73m² in treatment and 85.38±10.66 mL/min/1.73m² in control group (p= 0.371). Conclusion: Current study shows that magnesium supplementation did not prevent carboplatin-induced nephrotoxicity in cancerous children. Anyway, we propose magnesium oxide supplementation for such patients considering this fact that magnesium in an essential element for cell and tissue growth, maintenance and metabolism.