Hi, Mintu Sarker.
The quick answer is both are correct, but not everyone agrees that both are correct, and that's the problem. Here's the issue: the word "than" can be classified as either a conjunction or a preposition, and that's the root of the debate.
"Than" as a Conjunction
When "than" is used as a conjunction, it looks like this:
John is taller than I am.
John is taller than I.
(This is just a more succinct version.)
"Than" as a Preposition
When "than" is used as a preposition, it looks like this:
John is taller than me.
Grammarians have been arguing for hundreds of years over whether "than" is a conjunction or a preposition. Here's the bottom line. It is perfectly acceptable to write:
(This means the following are also acceptable: "than he," "than she," "than we," "than they.")
(This means the following are also acceptable: "than him," "than her," "than us," "than them.")
You can read more about this here