A thermal module includes a heat sink (400), a heat pipe (200) and a fastening device (300). The heat pipe (200) includes an evaporator (240) and a condenser (220) connected with the heat sink. The fastening device includes a base member (320) and a spring member (340). The base member includes a contacting plate (322) and two locating pins (324). Each locating pin includes a head portion (3240) and a neck portion (3244). The spring member includes a pressing portion (342) pressing the evaporator towards the contacting plate. Two locating holes (348) are defined in the pressing portion. The spring member is mounted on the base member via engagement of the locating holes with the locating pins. The spring member has a thickness smaller than a length of each of the neck portions and is movable along the neck portions to suit for a size of the evaporator.