PURPOSE: To improve the curability of a cold-curing epoxy resin compsn. for prepreg by mixing an epoxy resin with a cold-curing agent, dicyandiamide, a specific urea deriv., and a reactive thickener.

CONSTITUTION: The compsn. excellent in curability and having a viscosity at 50°C of 1,000-200,000cP is obtd. by mixing 100 pts.wt. at least one epoxy resin (e.g. a glycidyl ether or a glycidylamine) with a cold-curing agent having a reaction initiation temp. of 40-115°C in an amt. of 0.2-3.0 equivalents based on 1 equivalent of the resin, 1-10 pts.wt. dicyandiamide, 1-10 pts.wt. urea deriv. of the formula (wherein R and R' are each H, Cl, Br, lower alkyl, or lower alkoxy) [e.g. 3-(3,4-dichlorophenyl)-1,6-dimethylurea], and 0.2-3.0 pts.wt. reactive thickener (e.g. m-phenylenediamine).