Tutor, Jasmin Tumanda creator Xue, Wenchao Chairperson Visvanathan, C. Examination Committee Ho Huu Loc Examination committee Government of Japan Scholarship Donor text series technical report th Pathum Thani, Thailand Asian Institute of Technology 2021 continuing eng 80 leaves : ill. Studies on MP contamination in freshwater environments are relatively scarce compared to that of marine environments. This is especially true for freshwater environments in Asia. In this research, the MP abundance, spatial distribution, and variability were studied in order to gain a better understanding of MP occurrence in the Chao Phraya River, an important river system in Thailand. Samples of microplastics were taken using a device called Albatross along the lower Chao Phraya River covering the provinces of Pathum Thani, Nonthaburi, and Bangkok. To assess their vertical distribution, samples were also taken at 30 cm and 60 cm depths at the most upstream and downstream sites of the study area. Samples were pre-treated through wet peroxide oxidation and density separation. A microscopic examination was conducted to categorize MPs based on size, morphology, and color. MPs were analyzed through FTIR to identify their polymer types. MP abundance ranged from 2.92 to 35.53 n/m3 with a mean abundance of 9.84 n/m3 . Fibers were the most abundant upstream and fragments were most observed downstream. MP abundance were higher in more urbanized areas and on surface level. It tends to decrease as depth increases. Majority of the MP were small-sized (0.5-1 mm) and medium-sized (1-2.5 mm). Most MPs were found to be transparent and white and PET, PE, and PP were the most observed polymer types. This is the first study that comprehensively explored the vertical and horizontal distribution of size, shapes, colors, and polymer types of MP in the surface water level of the lower Chao Phraya River. Future studies may explore on other areas on the river to provide a more thorough understanding of MP contamination in the Chao Phraya River. A research study submitted in partial fulfillment of the requirements for the degree of Master in Marine Plastics Abatement Research studies project report (M.) - Asian Institute of Technology, 2021 Microplastics Thailand Lower Chao Phraya River Asian Institute of Technology. http://203.159.5.9/ait-thesis/detail.php?q=B16279 http://203.159.5.9/ait-thesis/detail.php?q=B16279 220428 20260817161346.0