The experimental results of high order harmonic generation in diatomic molecular N 2 are presented. The effects of different gas density, different laser energy and different laser polarization on harmonic emission and harmonic order are mainly studied in the experiment. At the same time, the results are analyzed theoretically. It is indicated that the confirmation of the optimal gas density and the incident laser energy are needed in order to get the stronger harmonic emission and to avoid the phase mismatch caused by density increasing of the free electrons.