An increase in N₂ fixation by diazotrophic organism due to increased stratification driven by climate changes, may decrease phosphate concentrations and result in P limitation in the oligotrophic upper ocean, which challenges the traditional view that nitrogen is generally the primary nutrient limiting phytoplankton productivity in oceanic waters. Which nutrient, N or P, is the most limiting nutrient for phytoplankton growth in the oligotrophic Pacific Ocean has been on debate over recent years. More studies on nutrient limitation are apparently needed to resolve this debate in the Pacific Ocean. In August and September of 2009, nutrient enrichment bioassays were conducted at two representative stations, N1 (160.58°E, 21.61°N) in the western Pacific Ocean with extremely low nutrient (below detect limit) and Chl a concentrations, and N2 (154.12°W, 10.12°N) in the eastern Pacific Ocean with shallower nutricline due to the influence of equatorial currents, in order to examine the spatial variability in the potential limiting nutrient for phytoplankton growth in the Pacific Ocean. Nutrients were added in 5 combinations in bioassays: control (no addition), NO₃+PO₄ (N+P); NO₃+SiO₄ (N+Si), PO₄+SiO₄ (P+Si), and NO₃+PO₄+SiO₄ (N+P+Si). The limiting nutrient was judged based on the response of algal biomass and nutrient depletion among treatments. Phytoplankton exhibited different response to nutrient additions at two study sites. Phytoplankton biomass increased dramatically in response to both N and P additions at station N1 where the concentrations of Chl a increased from 0.03 μg/L at the beginning to 2.12 μg/L and 1.83 μg/L in N+P+Si and N+P treatments at the end of incubation, respectively. However, the maximum Chl a concentration achieved in N+Si treatment was slight higher than that in and P+Si treatment. Furthermore, P was depleted before N and Si in the N+P and N+P+Si treatments at station N1. In contrast, algal biomass was significantly stimulated only when both N and P were added at station N2, where the concentrations of Chl a increased profoundly from 0.10 μg/L to 0.34 μg/L and 0.40 μg/L at the end in N+P+Si and N+P treatments, respectively, while they was not stimulated in the N+Si and P+Si treatments Furthermore, N always disappeared before P and Si in N+P, N+Si and N+P+Si treatments. These results showed that the limiting nutrient varied spatially during summer in the Pacific Ocean. N and P co-limitation occurred at both stations, with P being the primary limiting nutrient at N1 and N at N2. In addition, changes in the N∶P ratios during the incubation demonstrated distinct patterns, likely due to difference in the phytoplankton composition. N∶P ratios rose from 15.7 at the beginning to 59.2 at the end of the incubation at N1, while N∶P ratios decreased from 15.3 to 0.06 at N2. This implied that the uptake ratio of N∶P was lower than the Redfield ratio (16N∶1P) at N1, but higher than the Redfield ratio at N2. This might explain why P was the primary limiting nutrient in the western Pacific Ocean but N in the central Pacific Ocean. It is speculated that P limitation possibly is associated with N₂ fixation in the oligotrophic western Pacific Ocean.