Disclosed are a spin transistor and a method of operating the spin transistor. The disclosed spin transistor includes a channel formed of a magnetic material selectively passing a spin-polarized electron having a specific direction, a source formed of a magnetic material, a drain, and a gate electrode. When a predetermined voltage is applied to the gate electrode, the channel selectively passes a spin-polarized electron having a specific direction and thus, the spin transistor is selectively turned on.