To understand the nutritional quality of planktonic food for marine fish, the fatty acid composition of plankton collected from the Yellow Sea was investigated and compared between winter and spring. As soon as the plankton were collected at sea, they were size-fractionated into six groups:100, 200, 300, 500, 1000, and 2000 μm, and then immediately stored in liquid nitrogen. The fatty acid contents of the plankton were determined using capillary column gas chromatography in the laboratory. Compared with the spring-collected plankton, the winter-collected plankton in all size groups had higher total saturated fatty acid (SFA) contents, but lower contents of total monounsaturated fatty acids (MUFA), total polyunsaturated fatty acids (PUFA), and the fatty acids 16:1ω7, eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and 20:1+22:1. The different size groups of plankton collected in both winter and spring were compared. The contents of ω3PUFA, EPA, DHA, and 20:1+22:1 were higher in large plankton (>500 μm) than in small plankton (<500 μm). However, the contents of the fatty acids 16:0 and 18:0 were higher in small plankton than in large ones. Among the nutritional quality indicators (DHA/EPA, 16:1/16:0, and Σ16/Σ18 ratios), the Σ16/Σ18 ratio was higher in winter-collected plankton than in spring-collected plankton, while the DHA/EPA and 16:1/16:0 ratios were higher in spring-collected plankton than in winter-collected plankton. The DHA/EPA ratios reached to 2 in the 500 μm and 1000 μm groups in winter, but were greater than or close to 2 in all size groups in spring. Multiple analyses showed that the seawater temperature had the strongest effect on the fatty acid composition of plankton in the Yellow Sea, especially the MUFA and PUFA contents in plankton of the 100-1000 μm size groups. The results of this study show that winter-collected plankton of >500 μm could meet the nutritional requirements of marine fish, especially larvae and juveniles. Spring-collected plankton in size groups from 100 μm to 2000 μm, which have higher DHA/EPA ratio and ω3 PUFA content, are also suitable food sources for marine fish. It may be inferred that the size spectrum of plankton suitable as food for marine fishes is wider in spring than in winter in the Yellow Sea.