To get the National Dex in Pokémon FireRed and LeafGreen, you have to beat the Elite Four, catch 60 unique Pokémon species, and then talk to Professor Oak.
If you’ve already caught 60+ Pokémon before you beat the Elite Four, then Oak will meet you at your home after the credits play and he’ll walk you through a cutscene where he gives you and your Rival the National Dex.
Otherwise, you will need to catch 60 Pokémon after beating the game, and then go meet Oak at his lab to get the National Dex.
As mentioned, there are just two prerequisites to obtaining the National Dex:
- Defeat the Elite Four for the first time
- Catch 60 Unique Pokémon
The National Dex is basically an end-game reward for defeating the Elite Four and filling out your Pokédex. So after you’ve done that, just talk to Oak and you’ll get the new National Dex!
Getting the National Dex
The first step to obtaining the National Dex is defeating the Elite Four, and then defeating the Champion to become the new Champion.
If you’ve already beaten the Elite Four once, you can skip this step.
After you become the Champion, the end credits will play.
After the credits, you’ll be sent back to the main menu. From here simply load your game.
You’ll spawn at your home, and Professor Oak will greet you.
After talking to Oak, he’ll check your Pokédex to see if you’ve caught 60 Pokémon.
If you have, you’ll follow him back to his lab. If you haven’t, you’ll need to go catch Pokémon until you’ve caught 60 and then head back to his lab to talk to him.
A cutscene will play that shows Oak giving you and your Rival each a National Dex and sending you on your way.
What is the National Dex & What Does It Do?
Once you have the National Dex in FireRed/LeafGreen, you’ll be able to get Pokémon that were introduced in Generation two and three via trade.
You’ll also be able to evolve certain Gen I Pokémon that had their evolution introduced in Gen II.
For example, Golbat will be able to evolve into Crobat after you get the National Dex. Without the National Dex, these evolutions would fail or simply never happen.