Abstract: As a new proxy to reconstruct sea waterCO₃^2- in the past, shell weight of planktonic foraminifera can provide clues to revealing ocean carbon cycle. Shell weights of G. sacculifer measured from a tropical western Pacific sediment core WP7 during the last 250 ka show higher values in glacial periods and lower values in interglacial periods responding to changes in CO₂ concentrations in the Antarctic Ice Core of Vostok, except in MIS1 and MIS4. Hence, shell weight variations in this sea area are mainly controlled by the CO₂ concentration in atmosphere. As a result of intensified upwelling and carbonate dissolution, shell weights have reduced during MIS4. Shell weights also can be affected by coexisting symbionts, but not the changes in temperature and surface nutrient levels. Shell weights of G. sacculifer exhibit a generally inverse relationship with CO₂ concentrations, and thus they can be used as a reliable proxy to trace variations inCO₃^2- of surface water.