Disclosed is a peroxide monomer having the formula:CH.sub.2 =CR--A--OO--B (I)wherein R represents a hydrogen atom or an alkyl having 1 to 5 carbon atoms, A represents --NH--CO--, --CO--NH--CO--, or --CO--O--CH.sub.2 CH.sub.2 --NH--CO--, and B represents a moiety which has excluded the group (--OOH) from a hydroperoxide and a preparation thereof. The present invention also provides a polymer whose a main chain composed of carbon-carbon bonds is bonded with a pendant peroxide group represented by the formula:--A--OO--B (II)wherein A and B is the same as mentioned above, the content of the pendant peroxide group being 0.1 to 99.9% by weight and a molecular weight of said polymer being 1,000 to 100,000; and a preparation thereof.