PURPOSE: To produce a cyclopentadiene graft polymer in good yields by a simple operation, by graft-polymerizing a radical-polymerizable monomer with a polymerization solution of a thermal cyclopentadiene polymer.

CONSTITUTION: A mixture of about 95W40wt% cyclopentadiene monomer and about 5W60wt% comonomer is thermally polymerized at about 200W300°C for about 0.5W20hr (content of unreacted monomer ≤about 10wt%). A radicalpolymerizable monomer (e.g., ethylene. styrene, butadiene, or vinyl acetate) is added to the obtained polymrization solution, and the mixture is graft-polymerized in the presence of a radical initiator (e.g., benzyl peroxide) at a temperature of about 60W260°C for about 0.5W20hr.