Liquid epoxy resins (EP), curing agents, as well as other additives such as flame retardants are preferred to be formulated as one-pack materials rather than being mixed just prior to their applications. Therefore, they suffer premature curing with increasing of viscosity during storing and operating. To address the above challenging problem, we designed and synthesized a novel latent flame-retardant curing agent named 1-(diphenylphosphinyl)-1H-imidazole oxide (DPPIO) for EP, which was proved exhibiting not only long pot life during storing and fast curing as operating, but also excellent flame retardance after curing. It was observed from dynamic rheometry that, DPPIO/EP kept stable near room temperature for a long time but quickly gelated within only 6.5 min at 150 degrees C. With only 15 wt% addition of DPPIO, the limiting oxygen index (LOI) of the cured sample increased to 38.0% from 21.0% of the contrast sample, and UL-94 V-0 rating at 1.6 mm thickness was successfully achieved. Peak of heat release rate (PHRR) and total heat release (THR) obtained from cone calorimetry further certified excellent flame retardance of DPPIO/EP. Another typical flame-retardant group with different chemical environment was designed for modifying imidazole to obtain a derivative named diphenyl 1H-imidazol-1-ylphosphonate (DPIPP) in the same way. It was verified that DPIPP/EP showed acceptable latent curing efficiency and satisfactory flame retardance as DPPIO/EP did. Therefore, it's a generally effective and facile approach to develop latent flame-retardant curing agents for EP by modifying imidazole with appropriate flame-retardant groups. Taking advantage of the features, these one-pack flame-retardant epoxy materials can bring more chances for widespread applications.