A transformer includes a bobbin, a first magnetic core, a first and a second coils and a second magnetic core. The first magnetic core is disposed in the bobbin. The first coil and the second coil respectively wind around the bobbin. The second magnetic core covers at the exterior of the bobbin. A main magnetic flux is formed between the first magnetic core and the second magnetic core. A lamp driving system includes the above-mentioned transformer and a lamp. The first coil is input by a first voltage signal, the second coil is induced to generate a second voltage signal, and the lamp is driven by the second voltage signal.