A transformer includes a winding member, a magnetic core assembly and an insulating seat. The winding member includes a bobbin, a primary coil and a secondary coil. The primary coil and the secondary coil are wound on the bobbin. The magnetic core assembly includes a first magnetic core and a second magnetic core. The winding member is arranged between the first magnetic core and the second magnetic core. The insulating seat includes a receptacle, at least one ventilation hole, at least one sheltering member and plural pins. The winding member and the magnetic core assembly are accommodated within the receptacle. The ventilation hole is formed in the sheltering member. The first terminals of the primary coil and/or the second terminals of the secondary coil are connected with the pins.