To help understand characteristics of N2-fixation, N2-fixation rates, abundance and diversity of nitrogen fixers in the different water masses of the NW Pacific were examined during October 2007 and June 2008 cruises. The results showed that N2-fixation rates in the warm pool are very high, and the main contributor is Crocosphaera. The analyses also reflected variation of N2-fixation rates, diazotroph diversity and abundance in different water masses of the NW Pacific.