A bobbin has a body and a ring. The body has a seat, a spool and a winder. The seat has a taper surface with multiple stairs. The stairs have a guiding surface, and the guiding surfaces of all of the stairs face to a same direction. The spool is formed on the top of the seat. The winder is formed on the bottom on the seat. The ring is removably mounted on the winder to form an annular gap between the seat and the ring. A secured end of thread is wound around the annular gap. A free end of thread is wound around the seat and extends curvedly over one of the stairs. Therefore, the stairs each having one guiding edge prevent the thread from being broken.