According to Variety, Franco has joined the cast of the comedy along with Danny McBride. McBride also co-wrote the screenplay with Ben Best.
The story follows two spoiled and arrogant princes in a medieval fantasy world. When a magician casts a spell to kill their father, they must take over and begin a quest to save their family and kingdom.
Production is set to begin on July 20 in Belfast.
Franco and McBride worked together in the comedy Pineapple Express.
James Franco is currently shooting the drama, Howl, expected to be released in 2010.