The epoxy resin without hardener can harden by a ring-opening reaction in the presence of the alkalies produced by the hydration of cement in epoxy-modified mortars and concretes. This paper investigates the effect of curing conditions on the strength improvement of polymer-modified mortars using bisphenol A-type epoxy resin without hardener. The polymer-modified mortars using epoxy resin are prepared with various polymer-cement ratios, and subjected to ideal, water, dry and heat cures. In the heat cure, the epoxy-modified mortars are sealed or unsealed with a PVDC (polyvinylidene chloride) film. The epoxy-modified mortars are tested for flexural and compressive strengths at desired curing methods. The microstructures of the epoxy-modified mortars are also observed by scanning electron microscope. The effects of curing conditions on the strength development of the epoxy-modified mortars are examined. From the test results, the marked effectiveness of the heat cure under the PVDC film sealing against the development of the strength of the epoxy-modified mortar without the hardener is recognized. The compressive and flexural strengths of 1-day-$80^{circ}C$ heat-cured, PVDC film-sealed epoxy-modified mortars without hardener reach 23 to 30MPa and 6.4 to 7.8MPa respectively, and are about two times of unmodified mortar. Such high strength development of the epoxy-modified mortars may be achieved by the dense microstructure formation by cement hydrates and the hardening of the epoxy resin in the mortars.