The size reduction of waste polymer and composite materials has become an important subject in the field of waste reclamation. Energy consumptions in size reduction of waste polymers and composites as GFRP (Glass Fibre Reinforced Plastics) are very large for the reason of their high strength.Then in order to increase the pulverizing rate of polymers and GFRP, its dependence on boiling watercorrosion has been investigated for polymers (2 kinds of unsaturated polyester) and GFRP.The results obtained are as follows:(1) For the Polymers, pulverizing rate increased with increasing immersion time in boiling water.(2) For GFRP, on the aperture size of screen D=12-8mm, pulverizing rate displayed a minimum at a criticalvalue of immersion time (after about 12hr from start), and after that, increased rapidly, on D=6-4mm, thepulverizing rate increased from first.And it was found that the pulverizing rate depended on changes ILSS (Interlaminer shear strength) andspecific strength due to boiling water corrosion.