The invention relates to a process for the production of post-crosslinkable thermoplastic resins by bulk-polymerizing a polymerizable composition (A) comprising (I) a monomer fluid containing a cyclic olefin (.alpha.) having two or more metathetical ring-opening reaction sites in the molecule in an amount 10 wt % or above based on the total amount of the monomers or a monomer fluid containing a norbornene monomer and a crosslinking agent, (II) a metathetical polymerization catalyst, and (III) a chain transfer agent; thermoplastic resins obtained by this process; and a process for producing crosslinked resins or crosslinked resin composite materials which comprises laminating such a thermoplastic resin with a substrate at need and then crosslinking the thermoplastic resin. According to the invention, thermoplastic resins which are free from odor due to residual monomers and excellent in storage stability can be efficiently obtained by a simple process of bulk-polymerizing the composition (A). The process is not only easy and simple but also applicable to continuous production, thus being industrially advantageous. The crosslinked resins and crosslinked resin composite materials obtained according to the invention are excellent in electrical insulation properties, mechanical strengths, heat resistance, dielectric characteristics and so on, thus being useful as electrical materials or the like.