PURPOSE: An electrical insulating composition, consisting of a polyolefin, and a β- hydroxy oxime compound having a specific structure, suitable for power cables, etc., and having improved water treeing resistance.

CONSTITUTION: (A) 100pts.wt. polyolefin, e.g. low-density polyethylene, is incorporated with if necessary (B) 0.05W5pts.wt. crosslinking agent, e.g. dicumyl peroxide, and (C) 0.005W5pts.wt. β-hydroxy oxime compound expressed by formula I, preferably formula II, (R1 is H or aromatic ring; R2 and R3 are aromatic ring) or formula III (A, B, C and D are H, alkyl, aryl, etc.; formula IV is naphthalene or anthracene ring substituted by alkyl, aryl, etc.; R4 is H, alkyl or aromatic ring), e.g. salicylaldoxime, and (D) an additive, e.g. a crosslinking assistant, e.g. triallyl isocyanurate, and the resultant mixture is then kneaded and molded to give the aimed composition.