Abstract: Based on the grain size data acquired from 164 bottom samples, we studied the distribution pattern and transporting trend of the sediments in the Gulf of Thailand (GoT) using the Gao-Collins's method for sediments grain size analysis. In general, most of the sediments in the study area are poorly sorted, and the sorting coefficient varies from 1.3 to 2.9 with an average of 1.8 and the skewness varies from -1.8 to 2.5 (with a mean value of 1.2). According to the research results, the study area is divided into three provinces, i.e. the provinces 1, 2 and 3. The sorting and skewness in the province 1 are 1.73 and 0.91, and those in the province 2 and 3are 1.81, 1.33 and 1.91, 1.24, respectively. It means that fine-grained sediments (silt and mud) with mean grain size of 6.5Ф are widely distributed in provinces 1 and 2, while the coarse-grained sediment (silty sand) with mean grain size of 5.5Ф are mainly distributed in the cross area between province 1 and 2 and the near-shore area of province 3. The sorting of sediments from the coastal areas including northern-, middle- and southwestern parts of GoT is poorer than that from other regions. We suggest that sediment transport in the GoT is mainly controlled by three factors, namely rivers, ocean current and East Asian monsoon. In details, for the provinces 1, the sediment transport is controlled by rivers which take sediments from estuaries into deep-water areas. For the province. 2, ocean current dominates the sediment transportation in the peripheral part of the province to the center of GoT. As for the province 3, coastal current plays a critical role to move sediments away from the nearshore area. Under the influences of these three factors mentioned above, a sediment depocenter is formed around the Sumui Island.