West lake is larger than 500 ha, the lake is meaning for sightseeing tour of the local and foreign tourists. The Lake has not only great scientific significance but also plays an important role in climate regulation, regulating the amount of groundwater in Hanoi. Zooplankton and phytoplankton have a very important role in lake ecosystems because phytoplankton is considered as a major food source for the organisms of higher trophic levels in the aquatic food chain, and this autotrophic organism plays important role in biotransformation and distribution of arsenic species in the aquatic environment Bioaccumulation and biotransformation of arsenic by phytoplankton, and trophic transfer of arsenic in freshwater food chains have been important concerns because of possible human health effects of the toxic metalloid from dietary intake. Therefore, the study of arsenic levels in zooplankton and phytoplankton have a very important role in assessing water quality in the area and early warning for environment of the living creatures in the lake ecosystem. The research results at 9 sites in the West lake area showed that arsenic levels in zooplankton is 3.5 times higher than phytoplankton. Arsenic levels in zooplankton and phytoplankton seasonal changes and linear relation moderate and relatively close to the level of arsenic in lake water.