Abstract: To understand the composition and particle size distribution in spring and autumn in the northwest Pacific Ocean, east of Japan, LISST-100X instrument was used for the particle size distribution of suspended particulate matter（SPM）in the top 200 m surface water in May and September, 2015. The major component of suspended particulate matter was analyzed with Scanning Electron Microprobe（SEM）and Energy Dispersive Spectrometer (EDS). The results shows that the SPM in the study area is mainly composed of large particle size greater than 133μm and concentrated in certain layers. Concentration of SPM in autumn is much higher than in spring. The proportion of large particle size particles is decreased while the proportion of mediumsized particles increase and medium and smallsized particles distribution is more uniform in the water column <200m. Such trend is increasing with particle diameter. The main components of suspended particulate matter are single mineral particles, biological particles and flocs that mainly deprived from marine plankton and terrestrial material transport. Concentration and particle size distribution of the suspended particulates are controlled by biological growth, the continental input of aeolian sediment, dust and ocean current conditions.