grammar

No, that is not correct.

I knew it.

Patty is correct, but I wonder if you meant I did too know it.  If so, this is a casual emphatic expression used only in informal spoken English, and the more formal version would be I did so know it.

The meaning of the expressions I did too know it or I did so know it is to strongly disagree with someone who said you didn't know it.  It is so strong that it could be considered rude, if not a little childish.  Either expression is often shortened: I did too! / I did so! and usually punctuated by an exclamation mark.

A:  Jack told me you didn't know that the party is tomorrow.

B: I did too know it! / I did too! / I did so know it! / I did so!

I hope this helps.