Microplastic pollution is a global issue and a hot issue in Thailand. This is the first assessment of microplastics at Libong, a pristine island, located in Andaman Sea. The investigation was carried out by collecting the sediment from beach and mudflat areas of the island in May and July, 2019. Three class sizes of microplastics were determined: greater than 5 mm, 18 pieces; 1-5 mm, 28 pieces; and less than 1 mm, 129 pieces. The total number of microplastics from beach sediment was greater than that from the mudflat sediment. The discovered microplastics were mainly constituted of fibers (59%) and fragments (41%). A great variety of colors were found, and white (43% from beach, 41% from mudflat area) was the most common, followed by blue (9% from beach, 35% from mudflat) and red (12% from beach, 6% from mudflat). Polymers identified by Fourier Transform Infrared Spectrophotometer consisted of poly vinyl chloride (PVC), polypropylene (PP), Nylon, Polyethylene (PE), Polyester, Polyacrylate (PA) and polymer with a structure similar to EPDM rubber.