PURPOSE: To prepare a substrate capable of improving the recording and reproduction qualities of an optical recording media without the loss of mechanical and thermal properties by polymerizing a specific cycloolefin monomer mixture component in a specific embodiment in the presence of a catalyst and metharocene and processing the obtained polymer thermoplastically.

CONSTITUTION: The process for producing a substrate for a recording medium consisting of a copolymer which is produced by polymerizing without opening the ring 0.1-100 wt.%, based on the total amount of monomers, of at least one of monomers of the formulae I, II, III and the like, 0-99.9 wt.%, based on the total amount of the monomers of formula I, II, III of a cycloolefn of formula IV, and 0-99.9 wt.%, based on the total amount of the monomers, of at least one of acyclic 1-olefins of formula V at -78°C to 150°C and at a pressure of 0.01-64 bar, in the presence of catalysts and metharocene, comprises settling or dispersing the polymerized medium having a polymerization reaction stopped in liquid such as ketone and water, and subsequently filtering and drying the dispersed medium and processing thermoplastically this cycloolefin copolymer to form a disk.