This invention provides novel thermosetting preform having superior transparency which is composed of the resin produced by partially polymerizing the mixture of (A) 4 to 30 wt % of at least one vinyl monomer containing carboxyl group, (B) 4 to 96 wt % of at least one vinyl monomer containing glycidyl group, and (C) up to 92 wt % of at least one copolymerizable monomer, said preform being characterized in that the amount of unreacted monomer remaining therein is less than 20 wt % and the glass transition temperature range is 120.degree. C. or lower. This invention also provides a method of production of a highly transparent molded article with high productivity which is superior in heat resistance and in chemical resistance by heating and molding said preform.