An investigation of the distribution and size composition of chlorophyll a concentration was carried out in the Southwestern Indian Ocean (47-70°E, 25-40°S) in January 2011, and factors that might affect the distribution and size composition were discussed. Fluorometric method was used to obtain total and size-fractioned chlorophyll a concentrations. The survey was carried out during the austral summer, and there were 6 stations altogether, two of which located in the region of the south Indian Ocean Subtropical Gyre (IOSG) and the other four located in the subtropical convergence zone (SCZ). Water samples were collected from 13 layers in 0-200m column of each station and the vertical profiles of chlorophyll a, nutrients (NO₃^- 、PO₄^3- and SiO₃^2-), temperature and salinity were obtained. The objective of this study was to investigate the spatial distribution and size fraction of chlorophyll a concentration and to relate them to environmental factors. The results showed that the surface chlorophyll a concentration was low and did not exceed 0.07mg/m³, and the depth of the subsurface chlorophyll a maximum was deep, at approximately 100m and sometimes reaching as deep as 150m in the region of IOSG; However, in the SCZ, the surface chlorophyll a concentration was higher (0.164-0.247 mg/m³), and the subsurface chlorophyll a maximum layer occurred between 50-70m. In the region of IOSG, phytoplankton growth is limited by nutrients supply, while the frontal system formed by the Agulhas Return Current (ARC) and the Subtropical Convergence (STC) is a region of intense mesoscale activity presenting enhanced levels of biological production and chlorophyll a concentration. Based on the discussion of the factors affecting the distribution of phytoplankton, it appeared that NO₃^- was the main factor that limited the growth of phytoplankton in the southwestern Indian Ocean, and PO₄^3- could be the main limiting factor in some areas. Size-fractioned chlorophyll a concentration clearly showed that the study area was a typical oligotrophic open ocean. The picophytoplankton (0.2-2μm) chlorophyll a always dominated, representing 71.1% of the total chlorophyll a concentration on average, while the average contribution of nanophytoplankton (2-20μm) and microphytoplankton (>20μm) chlorophyll a to total chlorophyll a concentration were 24.2% and 4.7% respectively. The dominance of picophytoplankton was resulted from the low nutrient concentration environment because picophytoplankton were more competent over the microphytoplankton and nanophytoplankton under oligotrophic condition. There were some differences on the contribution of picophytoplankton to total chlorophyll a concentration between the regions of IOSG and SCZ, which were 77.9% and 66.7% respectively. The differences of the hydrographical characteristics, which in turn influenced the nutrient dynamics in the region of IOSG and SCZ, might lead to the significant difference of the spatial distribution and size composition of chlorophyll a concentration between these two regions.