Microzooplankton grazing accounts for more than half of the daily phytoplankton productivity, while its top-down control on nitrogen cycling such as phytoplankton nitrogen uptake in the upper ocean is nearly unknown. Here we for the first time combined traditional dilution method with nitrogen isotope labeling in the northwest Pacific ocean and china coastal sea. We found significantly grazing preference between different species of phytoplaknton in the in-situ grazer community. Moreover, such selective grazing significantly affect ammonium uptake of phytoplankton community but nearly no influence on nitrate uptake. Combining zero-dimentional box model and pigment data, we attribute such phenomenon to a possble selective grazing on ammonium utilizer. We imply that ammonium utiizer may be the preferred prey especially in the oligotrophic system.