Abstract: Seasonal and interannual variations of chlorophyll a in Bohai Sea remotely sensed by Moderate Resolution Imaging Spectroradiometer (MODIS) were investigated using 11-year averages of monthly data collected from 2003 to 2013.The chlorophyll a concentration in the surface layer was higher in costal zones,especially Laizhou Bay,Bohai Bay and Liaodong Bay.The concentration of chlorophyll a kept increasing during the last 11 years except for Laizhou Bay.There were 2 peaks each year,which were in spring and summer.The time that blooms occurring has a one month or two shift back and forth in different places.