Abstract: A transformer that comprises a coil module, a core and two bases is provided. The core penetrates through the coil module. The two bases are respectively disposed on the opposite ends of the core to clamp the coil module. The coil module comprises a plurality of coils which are sequentially stacked. Each of the coils includes a plurality of PCBs which are electrically connected and sequentially stacked. Each of the PCBs is formed with a printed circuit in a spiral configuration with respect to the core. Each of the coils is formed by the electrically connected print circuits which are formed on the adjacent PCBs. Thus, the present invention provides a transformer with high efficiency and slimness.