The 35-year-old actress played Lady Mary Crawley in the hit period drama from 2010 until the show's ending in 2015, reported E! online.
When quizzed about a potential reunion, she said, "I don't know when, but I'm hopeful."
The actress says she would be delighted if the cast gets back together for a big screen outing because she still misses the show and the friends she made during her five years on the programme.
"I miss the show and my friends on the show."
"We've been asked to keep ourselves available for dates in the future. But nobody has seen a script," he said.