To examine fluctuations in the fishery resources of the inshore waters off northeastern Taiwan, a long-term investigation and monitoring on planktonic copepods (Crustacea: Copepoda) may be one of the major measures. Plankton samples for copepod studies were collected during four seasonal cruises on board the research vessel Hai-Chien on January 7~9, April 20~22, July 22~24, and October 21~23, 2004. Samples were collected using a ring trawl net with a 335-μm mesh size at four fixed stations along a transect extending eastward from Ilan Bay to about 40 nautical miles offshore.In total, 128 species of copepods belonging to four orders, 25 families, and 49 genera were identified. Five species, i.e., Canthocalanus pauper, Cosmocalanus darwini, Clausocalanus minor, Oithona plumifera, and Oncaea venusta, were the most widely distributed species that occurred at all stations in each season. Twenty-eight species occurred only once throughout the year. The primary dynamic results showed that copepod abundances ranged from 345 to 1334 individuals (ind.)/m3 in continental shelf waters and from 51 to 341 ind./m3 in the waters of the Kuroshio Current, but numbers of species were higher in the Kuroshio waters (41~66) than in continental shelf waters (29~58), excluding results in autumn. The species richness increased and the biomass decreased seaward along the transect. Significant seasonal fluctuations in the dominant species composition were observed at all stations, with seasonal replacement rates of between 47.4% and 60.0%. Certain species, e.g., Calanus sinicus, might be considered an indicator species of intrusion eddy waters of the East China Sea into Ilan Bay and its adjacent waters.