Phytoplankton biomass and primary production were studied in the Bay of Bengal (BOB) to better understand the element cycle and energy flow in the northeastern Indian Ocean. The northeastern Indian Ocean cruise was conducted by " Shiyan I " RV from April 23 to May 2, 2010, initiated by South China Sea Institute of Oceanology, Chinese Academy of Science. The seawater surface temperature in the BOB was higher than that of the South China Sea, and the salinity showed great variation. Stratification phenomena were prevalent in the water column during the study period. Dissolved inorganic nitrogen (NO^-₃+NO^-₂) (<1 μmol/L) were deficient in the upper layer, and increased intensively below the euphotic layer. Surface chlorophyll a concentration was lower than 0.1 mg/m³, and the maximum value were mostly appeared at 75 m depth. Integrated chlorophyll a concentration in the water column ranged from 14.7 to 27.0 mg/m², and the high value was found at 88°-89° (E) adjacent area. Primary production in the surface water was lower than 0.2 mgCm^-3h^-1, and the maximum value was found between 50 m and 75 m. Assimilation number in the water column was used to calculated Integrated primary production, with the mean value of 1.44 mgC mg^-1 Chl a h^-1. Integrated primary production ranged from 199 mgCm^-2d^-1 to 367 mgCm^-2d^-1 in the BOB during the study period, with the high value occurred at 88-89° (E) adjacent sea. Pico-phytoplankton (44.3%) was the main contributor to the carbon fixation, followed by micro-phytoplankton (29.6%) and nano-phytoplankton (26.1%), despite that phytoplankton community structure varied with depth. Low values of DIN: P and DIN: Si indicated that nitrogen might limit the phytoplankton growth in the upper water of the BOB. The Arabian Sea and the BOB have different geographical characteristics and various physical dynamics. In the Arabian Sea, both vertical mixing in winter and upwelling in summer carried the nutrients into the euphotic layer and could stimulate the phytoplankton growth. In the BOB, stratification impeded the nutrients moving upward, and the cloudy weather could also decrease the light penetration, so the primary production in the BOB was lower than that of the Arabian Sea.