The openly-bisexual singer admitted she has a crush on the 'I Kissed A Girl' hitmaker - who is married to Russell Brand - as she's "so fit".
She said: "Who's hotter out of Russell and Katy? Katy! She's so fit, and has the most amazing face and boobs."
The 'Price Tag' hitmaker also revealed she used to get bullied at school, and still has to put up with receiving hurtful comments on social networking site twitter.
She added to more! magazine: "It wasn't horrific but it is tough to be called names. These days people have said, 'Come to my party!' I'm like, 'Wait a minute, the last time I saw you was years ago, when you were calling me names.' That sticks with you.
"I shouldn't have to deal with those kind of comments on twitter, and no one else should either. I'm not going round hurting people. I'm just trying to make a difference with music."