Ruby how to get natinaldex?

To obtain the National Dex in Pokémon Ruby, you need to complete the main storyline by defeating the Elite Four and the Champion, Steven. Afterward, visit Professor Birch in Littleroot Town; he will give you the National Dex as a reward for your achievement. Make sure to have seen all 202 Pokémon available in the Hoenn Pokédex to unlock it.