A flat light source including a first substrate, ribs, a phosphor layer, a second substrate, electrode patterns and an insulating layer is provided. The ribs are disposed on the first substrate. The phosphor layer is disposed on the surface of the ribs. The second substrate is located above the first substrate. The electrode patterns are disposed on the second substrate, and each electrode pattern is aligned to one of the rib correspondingly. The insulating layer covers the surface of the electrode patterns. In particular, an inert gas is filled between the first and second substrates, and a discharge path is formed between the adjacent electrode patterns above the phosphor layer.