The couple met while working together on 'The Last Song' in 2009 and Liam has previously admitted while he wanted to be professional on the film, they couldn't help but fall in love after working so closely together.
He said: ''What happened happened, and we've been together since.
''She makes me really happy. When you start, you want to be professional, but when you're filming those scenes with someone and pretending to love them, you're not human if you don't feel something.''
But, what a secret behind her slim body performance. Miley, 19, currently pays her trainer $200 per hour but when she travels, she flies him first class so she can exercise whenever she wants.