Abstract: Samples of chlorophyll a were collected in adjacent water of Yalu River estuary in August 2016 to discuss the distribution of sized-fractionated chlorophyll a and its influencing factors.The results showed that the total chlorophyll a concentration ranged from 2.91 μg/L to 17.46 μg/L, with an average value of 6.54±3.56 μg/L.The concentration of chlorophyll a in surface layer was much higher than it in bottom.The horizontal distributions of concentration of Micro-sized chlorophyll a in surface and bottom layer showed analogous tendency in general, which value reached to peak at the estuary of the river.Furthermore, the pattern of concentration distributions horizontally of Pico-sized chlorophyll a in bottom layer was similar to surface layer, with a degree of content gradually decreasing from northeast to southwest.However, the high value zone of the Nano-sized chlorophyll a in surface is situated on the west of the river estuary, with a horizontal distribution of north high and south low at bottom.As primary contributor the average contribution rate of Micro-sized chlorophyll a was up to 51.10% followed by Pico-sized chlorophyll a (27.61%) and Nano-sized chlorophyll a (17.29%).The principal component analysis (PCA) between size-fractioned chlorophyll a and environmental factors indicated that the concentration of underlying size-fractionated chlorophyll a was positive related to all environmental factors (p < 0.05).Moreover, the salinity, transparency were negative connected with size-fractionated chlorophyll a, whereas the content of nitrate, nitrite, phosphate, dissolved inorganic nitrogen (DIN), suspended substance and ratios of dissolved inorganic nitrogen (DIN) to phosphate could positively affect the structure of superficial size-fractionated chlorophyll a (p < 0.05).The concentration of Micro-sized chlorophyll a showed extremely positive correlation with nutrients than Nano-sized and Pico-sized chlorophyll a.