In this study, we evaluated a flame-retardant hardener for epoxy resins and a flame-retardant resin consisting of the same components, which can be applied to materials used in the aerospace, automotive, and construction industries. To overcome the predicaments associated with curing agents such as 9,10-dihydro-9-oxa-10-phosphaphenanathrene-10-oxide (DOPO)-dicyandiamide (DICY) that are difficult to synthesize successfully, we provided novel flame-retardant hardeners by attaching the phosphorous-containing group of DOPO to a carbon of DICY. We also provide a method beneficial for mass production of the flame-retardant hardener for epoxy resins. To measure the characteristics of the flame-retardant hardener, carbon fabrics were preimpregnated with these epoxy resins to form a halogen-free flame-retardant thermosetting carbon fiber prepreg. Then, samples of carbon fiber laminate with eight layers of prepregs were pressed at 180 ℃ under thermosetting conditions. The tensile, compressive, flexural, interlaminar shear, and in-plane shear properties of the carbon fiber laminate were measured in accordance with American Society for Testing and Materials (ASTM) D3039, D3410, D790, D2344, and D4255 standard testing protocols, respectively. The results indicate that the proposed material provides high strength and stiffness. Finally, according to the flammability rating of the UL-94 standard method, all samples were determined to be V-0. The results demonstrate that a flame-retardant hardener for epoxy resins could be mixed with resins to produce a standard qualified flame-retardant resin based on the UL-94 standards.