The major neurodegenerative diseases are characterized by increasing of activated-microglial cells and inflammatory cytokines in the central nervous system. Carrageenan extracted from red algae is a kind of polysaccharide with sulfate groups. The oligosaccharides were obtained from carrageenan by enzymatic degradation. To detect the immunomodulatory activity of κ-carrageenan oligosaccharides (KOS) on microglial cells and the relationship to the sulfate group content, the desulfated derivatives of KOS (DSK) were obtained by dimethyl sulfoxide-methanol-pyridine method. KOS was labeled with fluorescein isothiocyanate. The effect of KOS and DSK on lipopolysaccharide (LPS)-activated microglial cells was detected. Hematoxylin-eosin staining and flow cytometric were used to detect the cell viability. The "scratch" migration assay, ornithine analysis and RT-PCR were used to determine the cell migration, arginase and TNF-α released by microglial cells, respectively. The effect of LPS and KOS on microglial cells was determined by flow cytometry and laser scanning confocal microscopy. The results showed that KOS and DSK could inhibit the cell viability, arginase and TNF-α released by LPS-activated microglia cell with concentration dependent manner. But the effect of DSK was weaker than that of KOS. KOS aggregated on the cell surface firstly, and then they enter into the cell to the nucleus, spread over the entire cell finally. But the exist of LPS could prevent the entrance of KOS. It could be concluded that KOS could protect microglial cells from being activated by LPS, and its inhibition function had relationship to the sulfate group content of KOS, while there were competition between LPS and KOS.