This journal article shows the effect of cooling with liquid nitrogen on a gallium nitride (GaN) film that is exposed in argon plasma. We evaluated the damage induced by plasma on a n-GaN film (2µm) with photoluminescence (PL). Our preliminary result shows that the PL intensity less than 450nm stayed almost constant only when the film was cooled down with liquid nitrogen. This can be an indication that the damages induced by plasma can be avoided for the wavelength less than 450nm by cooling the film.