The 18-year-old singer - who has previously dated Wilmer Valderrama and Joe Jonas - admits the blonde Irish singer in the boy band is her favorite member.
Asked in a web-chat who her celebrity crush is she named Niall and said: "He's so adorable."
Blonde Niall has previously revealed he thinks Demi "seems to be a nice girl".
Meanwhile, Harry Styles - who sings in the band alongside Niall, Zayn Malik, Louis Tomlinson and Liam Payne - has revealed he is terrified of losing his infamous curly locks.
He told The Sun newspaper: "I am a little concerned I might go bald soon. It worries me it might fall out. I am just watching how much I put on it now."
Simon Cowell, who put the boy band together on the UK version of 'The X Factor', has previously claimed he is so excited about One Direction's success he is unable to sleep.
Louis revealed: "He's really, really excited about the reaction we've had out here and has said he's not sleeping properly because he keeps thinking about us and how it's going. He's far too excited to sleep. Which is quite cool."