An ion source using a field emission device is provided. The field emission device includes an insulative substrate, an electron pulling electrode, a secondary electron emission layer, a first dielectric layer, a cathode electrode, and an electron emission layer. The electron pulling electrode is located on a surface of the insulative substrate. The secondary electron emission layer is located on a surface of the electron pulling electrode. The cathode electrode is located apart from the electron pulling electrode by the first dielectric layer. The cathode electrode has a surface oriented to the electron pulling electrode and defines a first opening as an electron output portion. The electron emission layer is located on the surface of the cathode electrode and oriented to the electron pulling electrode.