A light-emitting device includes a pixel having a transistor provided over a substrate, and a light-emitting element. The transistor includes a single-crystal semiconductor layer which forms a channel formation region, a silicon oxide layer is provided between the substrate and the single-crystal semiconductor layer, a source or a drain of the transistor is electrically connected to an electrode of the light-emitting element, and the transistor is operated in a saturation region when the light-emitting element emits light. Further, in the light-emitting device, a gray scale of the light-emitting element is displayed by changing a potential applied to the gate of the transistor.