A one-year-old baby with a fever of 39°C that lasts overnight is considered to have a high fever. Whether it is serious or not depends on the specific situation of the child and cannot be generalized. Fever is an important clinical manifestation in the course of disease progression. If the fever exceeds 39℃ and lasts all night, if the baby has a normal complexion. And there are no other uncomfortable symptoms, such as headache, vomiting, abdominal pain, diarrhea and other symptoms, there is generally no serious problem, but we should pay more attention to observe the state of the child. However, if the baby continues to have a high fever that does not subside, accompanied by vomiting, confusion and other adverse reactions, the condition is more serious, prolonged high fever is likely to cause dehydration of the body, may also lead to damage to the function of various organs, serious cases will appear febrile convulsions, and even life-threatening, need to pay attention to. It is recommended to reduce the fever as soon as possible, giving the baby more water and physical cooling, such as a warm bath or wiping the baby’s neck, armpits and groin with warm water and other parts of the large blood vessels travel. If the fever persists, the baby should be taken to the pediatrician immediately.