Abstract: The present invention provides an osmium oxide composition comprising an osmium oxide microencapsulated in an aromatic polyolefin (hereinafter abbreviated as MCOsOx), a method for preparation of MCOsOx, which comprises allowing an osmium oxide to contact with an aromatic polyolefin in an organic solvent, and precipitating MCOsOx, an oxidizing agent comprising MCOsOx, a method for preparing a chiral diol compound, which comprises reacting MCOsOx, a chiral ligand and an olefin compound with each other, and a method for preparing a chiral diol compound, which comprises oxidizing an olefin compound with MCOsOx, wherein a chiral ligand further coordinates to an osmium oxide.