A thermal module includes a heat sink (20), a heat pipe (10) and a fastening device (100). The heat pipe has a condenser section (14) connected with the heat sink. The fastening device includes a base member (50) for fixing an evaporator section of the heat pipe, a positioning pole (58) disposed on and connected to the base member, and an elastic member (30). The positioning pole includes a neck (580) and a head (582). The elastic member includes an abutting portion (32) defining therein a positioning hole (320), and two locking portions (34) extending from the abutting portion. The neck of the positioning pole is freely and loosely received in the positioning hole of the abutting portion. A maximal outer diameter of the head of the positioning pole exceeds a diameter of the fixing hole.