Supposedly what are the journal entry of income summary?
Is it an asset?
Liability? Or
Owner's equity?

This is a temporary Account and used to close Income statement accounts to Balance Sheet accounts.

The difference in debits and credits in the Account is posted to the Retained Earnings Account.

For example revenues are credited to the Income Summary Account and Expenses are Debited to the Income Summary Account. The Difference is then added to Retained Earnings.

But, in warren's book. I found that income summary is part of number 3 account (Owner's Equity).

I realize this is old... but if you can pull up a thread over a year old, certainly I can post an answer almost a month later.

Pready never said it wasn't part of the equity, which under a typical numbering system would be 3. There was no mention of numbering systems and it can be done other ways. Retained earnings is part of equity.

So there's no "but" here. Absolutely nothing incorrect was said.