A infant nearly two years old was admitted to a Tien Giang Province hospital on Friday with many burns on his face.
Initial investigations suggest that the burns were caused by the boy's aunt, Tran Thi Thu, also known as Van Thi Kim Lien.
Thu, said to suffer from epilepsy and severely bad hearing, is said to have pressed a hot iron bar on the forehead, cheeks, chin and arms of her nephew, Tran Van Luc, who is just 20 months old.
Each burn mark was round two to four centimeters long and the torture had also caused the baby to have fever, hospital doctors said.
The baby's father, Tran Van Lap, said he worked far away from home on Thursday, so the baby's grandmother brought him to her house, where she lives with the aunt.
The grandmother said she was out in the garden when she heard the baby crying, so she ran inside and found the baby with burn marks on his face.
Police in the Mekong Delta province are going to test the aunt's mental condition and take further action in the case.
No information was given about the baby's mother or her whereabouts.