A transformer includes a bobbin, a primary winding coil, a secondary winding coil, a case and a magnetic core assembly. The bobbin includes a winding member, a first channel, and multiple ground pins. The ground pins have first terminal parts protruded from a surface of the winding member. The primary winding coil and the secondary winding coil wound around the winding member. The case includes a receiving portion for partially accommodating the winding member therein, a second channel communicated with the receiving portion, and multiple perforations corresponding to the ground pins. The first terminal parts are penetrated through the perforations and protruded from a surface of the case when the winding member is accommodated in the receiving portion. The magnetic core assembly is partially embedded into the first channel and the second channel, so that the magnetic core assembly is contacted with the first terminal parts of the ground pins.