Low residual resistivity in superconducting thin films is required for their applications in radio frequency (RF) cavities. Here we report on clean epitaxial MgB 2 films fabricated by the ex situ annealing of B films, grown by chemical vapour deposition, in Mg vapour. The films show sharp superconducting transitions at about 40 K and a high residual resistivity ratio of about 10. The result indicates that a clean precursor B film and a contamination-free annealing procedure are important for a viable MgB 2 film fabrication process for RF cavity applications.