Gut microbiome plays important role in animal nutrition by extracting energy and nutrients from other indigestible food. Previous studies have revealed diet as one of the key factors shaping gut microbiome composition and function. However, in contrast to the numerous lab-based studies on diet-gut microbiome dynamics, few studies investigate the causes and consequences of microbiota variation in wild. Since wild animals consume distinct diet throughout the year in response to seasonal fluctuation in food distribution and abundance, gut microbiota and its associated function thus may change accordingly. This study aims to investigate the seasonal variation in gut microbiome of wild Japanese macaques inhabiting Yakushima lowland. We directly examined the fermentative ability of gut microbiome by conducting in vitro digestibility assay monthly. Here we report the preliminary result regarding the seasonal variation in fermentative ability of macaques’ gut microbiome and discuss the relationship of gut microbiome function with seasonal dietary change.