How do you evolve doduo?

Train your Doduo to level 31 and it will evolve into a Dodrio.

Just to help, if you go to Kanto, follow the path and after the last fisherman, keep going and you'll see grass, lvl. 30 Doduo are in there!, also in soulsliver and heartgold go to kanto near victory rd there is patches of grass you can also catch doduo there lvl 30