A new glyceroglycolipid, ishigoside (1, 2-di-O-palmitoyl-3-O-(6-deoxy-6-amino)-${alpha}$-D-glucopyranosyl-glycerol, 1), along with two known compounds were isolated from the brown alga Ishige okamurae. The structure of the new compound was determined on the basis of spectroscopic analysis, including 1D and 2D NMR, MS techniques and chemical methods. Moreover, direct free radical scavenging activities of ishigoside were investigated by electron spin resonance spectrometry (ESR) technique. The results suggested that ishigoside was a potential free-radical scavenger against DPPH, hydroxyl, alkyl, and superoxide radicals with the $EC_{50}$ of 31.2, 16.7, 22.7, and $26.8{mu}M$, respectively.