We report the effect of epitaxial growth and lattice mismatch on the enhancement of saturation magnetization (M(s)) of ferromagnetic gamma(')-Fe(4)N thin films deposited on different single crystal substrates having lattice mismatches from 0% to 11%. It was found that M(s) in the gamma(')-Fe(4)N film increased with increasing degree of epitaxy and minimizing lattice mismatch between the film and the substrate. Maximum saturation magnetization of 1980 +/- 20 emu/cm(3) (about 24% higher than previous result) was obtained with LaAlO(3)(100) substrate having zero lattice mismatch after postannealing of 30 min, which is believed to originate from magnetovolume effect in well-ordered epitaxially grown films.