The spatial and temporal variability and size fractionation of chlorophyll a(Chl a) were investigated in the tropical and subtropical Pacific Ocean during four survey cruises from 2005 to 2009.The surface Chl a(S-Chl a) concentration ranged from 0.002 to 0.497 mg/m 3 and was obviously higher in the eastern Pacific than in the western and central Pacific.The vertical distribution of Chl a displayed a single peak pattern,and the maximum Chl a layer(MCL) was observed at a shallower depth in the eastern Pacific than in the western Pacific.All three size fractions of Chl a measurements in the surface water showed a similar distribution to total Chl a and were found in higher concentrations in the eastern Pacific than in the western and central Pacific.Picoplankton dominated the phytoplankton in the surveyed tropical and subtropical Pacific Ocean.Furthermore,pico-Chl a(0.2-2 μm) accounted for a larger percentage of the total Chl a in the central Pacific than it did in the western Pacific and eastern Pacific.In the western Pacific,there seemed to be a latitudinal variability in the phytoplankton community composition where small-sized phytoplankton(<2 μm) were more dominant in the tropical than in the subtropical western Pacific.The spatial and temporal variability and size fractionation of Chl a were controlled by hydrological and chemical characteristics and climate events,such as El Nin o and La Nin a. The spatial and temporal variability and size fractionation of chlorophyll a (Chl a) were investigated in the tropical and subtropical Pacific Ocean during four survey cruises from 2005 to 2009. Surface Chl a (S-Chl a) concentration ranged from 0.002 to 0.497 mg / m 3 and was obviously higher in the eastern Pacific than in the western and central Pacific. The vertical distribution of Chl a displayed a single peak pattern, and the maximum Chl a layer (MCL) was observed at a shallower depth in the eastern Pacific than in the western Pacific. All three size fractions of Chl a measurements in the surface water showed a similar distribution to total Chl a and were found in higher concentrations in the eastern Pacific than in the western and central Pacific. Picoplankton dominated the phytoplankton in the surveyed tropical and subtropical Pacific Ocean. Futuremore, pico-Chl a (0.2-2 μm) accounted for a larger percentage of the total Chl a in the central Pacific than it did in the western Pacific an d eastern Pacific.In the western Pacific, there seemed to be a latitudinal variability in the phytoplankton community composition where small-sized phytoplankton (<2 μm) were more dominant in the tropical than in the subtropical western Pacific. The spatial and temporal variability and size fractionation of Chl a were controlled by hydrological and chemical characteristics and climate events, such as El Nin o and La Nin a.