Mn and N codoped ZnO (or Zn1-xMnxO:N) films were grown on Si(001) and quartz substrates via reactive electron beam evaporation in the NH3 atmosphere. The growth temperature was 300℃. The as-grown Zn0.88Mn0.12O:N film was cut into two pieces and one of them was then annealed in O2 atmosphere at 400℃ for 30 min. X-ray diffraction measurements reveal that Zn0.88Mn0.12O or Zn0.88Mn0.12O:N films have the single-phase wurtzite structure. The (002) diffraction peak of Zn0.88Mn0.12O:N film is located at a smaller angle and has a wider line width compared with that of Zn0.88Mn0.12O film. Hall measurement shows that the as-grown Zn0.88Mn0.12O:N film is the n-type and changes to p-type after annealing. DC magnetization analysis shows that the as-grown Zn0.88Mn0.12O:N films are ferromagnetic at room temperature (RT) but the ferromagnetism (FM) is weak and unstable. However, the FM of Zn0.88Mn0.12O:N film at RT is remarkably enhanced and its stability efficiently improved by annealing in O2 atmospher. The origin of FM of Zn0.88Mn0.12O:N film at RT, as well as the mechanisms of its FM enhancement and stability improvement by annealing are discussed.