A fixing device and an image forming apparatus including the same are provided. The fixing device includes a rotatable endless belt passing through a fixing nip that is formed by a nip forming member and a backup member, and a first guide positioned inside the endless belt to guide the endless belt entering the fixing nip, wherein the first guide includes a first bent portion disposed at an upstream side of the fixing nip to support the endless belt and a second bent portion disposed at an upstream side of the first bent portion to support the endless belt, and a height of the second bent portion from the fixing nip is within a range of about 4% to about 10% of a height of the endless belt when being freely transformed.