Background: The use of radiation therapy for medulloblastoma can affect children’s visual system. We estimated children’s visual system complication probability in the craniospinal irradiation (CSI) technique with three-dimensional conformal radiotherapy (3D-CRT). Materials and Methods: CSI of fifteen medulloblastoma patients and a phantom were planned with 6 MV photon beams and 23.4 Gy prescribed dose. The doses of lenses were measured using thermoluminescence dosimeters (TLD). The delivered doses and complication probabilities were calculated based on the equivalent uniform dose (EUD) model to each contoured organ, including the bilateral lenses, optic nerves, retinas and optic chiasm. Results: The received dose for each organ was less than the tolerance value (p<0.001), except for the eye lens. The normal tissue complication probability (NTCP) values for all of the organs at risk (OAR) were found insignificant. The discrepancies of calculated and measured doses for the right and left lenses were 6.35% and 6.23% (p<0.001), respectively. Conclusion: The results of this study showed based on the International Commission on Radiological Protection (ICRP) publication 118 that children with medulloblastoma cancer treated with CSI with 3D-CRT method are susceptible to cataract complication.