The Jumbo flying squid (Dosidicus gigas), one of the key abundant fisheries resources, is widely distributed in the eastern tropical Pacific Ocean off Costa Rica. The first survey of D. gigas in this area was carried out in the middle of 1990s by the Japanese squid jigging vessels,followed by the development of a small scale commercial fishery. A survey of D. gigas was conducted by Chinese squid jigging vessels to evaluate the distributions of fishery resources and fishing grounds and to understand the relationship between fishing ground and environmental variables including biotc and abiotic environment. In this paper, based on the zooplankton data sampled by the Chinese squid jigging vessels in the waters (4°30' -10°24'N, 91°20'-100°00'W) of the eastern tropical Pacific Ocean off Costa Rica from July to August in 2009, we analyzed the species composition, abundance and spatial distribution of zooplankton to evaluate potential mechanisms of forming the D. gigas fishing grounds. This study showed that there were a wide range of zooplankton species in the survey area, including jellyfish (Hydroidomedusa, Siphonophorae, Scyphomedusae), Ostracods, Copepods, Amphipoda, Mysidacea, Euphausiids, Macrura, Brachyura, Sagittoidea, Caudata, Thaliacea, Polychaetes and Pteropods. In total, 13 categories and 115 kinds (excluding cephalopods larvae, fish eggs and larvae and pelagic larvae) were recorded, There were 23 species 17 genus 10 family 8 order in the Coelentera, 74 species 43 genus 30 family 8 order in the Crustacea, 4 species 1 genus 1 family 1 order in the Chaetognatha, 5 species 5 genus 3 family 3 order in the Urochordata, 6 species 5 genus 3 family 1 order in the Annelida, 3 species 3 genus 1 family 1 order in the Mollusca and other species including paralarvae and juvenile of fish and cephalopod and pelagic larva. The zooplankton species of high abundance included Copepoda and Sagittoidea, while Copepoda, Sagittoidea, Macrura and paralarvae and juvenile of fish and cephalopod were those occurring most frequently in the sample. The average biomass and abundance of zooplankton were (124.78±176.83)mg/m³ and (848±1219) ind/m³, respectively. Copepoda had the highest abundance reaching 727 ind/m³ followed by Sagittoidea which was 373 ind/m³ in abundance. It was found that the high yield of squid was distributed in the area with high abundance of zooplankton, and related to the copepods distribution. Distribution of cephalopods larvae is closely related to the Copepods and Sagittoidea. During the survey, the composition of zooplankton and its biomass were found to be influenced by the mound of cold water. Thus the composition and distribution of zooplankton in different seasons and its relationship with cold water mound require more studied in future with research efforts focusing on collecting more data over long time period.