A hair warming tool is in the form of sheet and has at least one heating part in the center thereof and a margin around said heating part. It is designed to wrap tied hair in said heating part and to be held in place to keep the wrapping state by fastening the margin with a fastening means such as an elastic band, a string or a wire. A hair treating method comprises wrapping tied hair with a hair warming tool of sheet form having at least one heating part while leaving at least the part of the hair near the roots unwrapped to warm the hair.