we studied on chemical & physical variation of cotton waste according to covering materials during outdoor fermentation for oyster mushroom bed culture with cotton waste medium. P.E vinyl and nonwoven thin cloth were used for the covering material. Temperate in the medium ascended up to 75℃ in treatment using nonwoven thin cloth but 60℃ in P.E vinyl treatment for fifth day to seventh day, after that time fell slowly. Before the fermentation, pH of cotton waste medium was 7.2∼7.5 but during the fementation, pH in using nonwoven thin cloth treatment was about 8.9 and pH in P.E vinyl fell 5.1. Ammonium ion concentration in cotton waste medium was 70-85ppm in treatment using P.E vinyl and higher than nonwoven thin cloth. Total carbon content of cotton waste medium was increased but total nitrogen content was decreased while fermentation make progress. when we investigated cotton texture wholly femented by SEM & TEM, cell wall of cotton in treatment using nonwoven thin cloth mostly degraded but it in P.E vinyl was kept unimpaired