A 22 month old child with thalassaemia major received unrelated umbilical cord blood transplantation. She was born to mother of HBsAg carrier and received hepatitis B immunoglobulin at birth and hepatitis B vaccination. She was HBsAg negative and anti‐HBs positive before transplantation. After transplant, she was taken care by her mother and found to be HBsAg positive at 2 year post‐transplant. Genotyping of the mother's and child's HBV status confirmed to be of same genotype and demonstrated horizontal transmission in post‐transplant setting. Passive immunization of HBV may be considered in early post‐transplant phase to prevent horizontal transmission of HBV, and antiviral treatment of the carer should be offered to prevent transmission of infection to immunocompromised child.