A composition for a wire coating member having flame retardancy that is heat resistant and productive without using electron irradiation crosslinking, and an insulated wire and a wiring harness having the same. The insulated wire includes a wire coating member containing the composition for the wire coating member that contains water-crosslinkable polyolefin containing polyolefin that is modified by a silane coupling agent, unmodified polyolefin, modified polyolefin that is modified by a functional group, a bromine flame retardant, a cross-linking catalyst, a phenolic antioxidant, and either one of a zinc sulfide, and a zinc oxide and an imidazole compound. The insulated wire is prepared by extrusion-coating a conductor with the composition to form the wire coating member around the conductor, and the wire coating member is water-crosslinked.