<p>A reactive oligomer for epoxy resin modifiers containing a flexible chain and imide groups is proposed. The effects of introducing imide groups into the reactive oligomer and the curing conditions of the epoxy resin with respect to the phase-separated structure and properties of the cured epoxy blends are investigated. Compared with an oligomer without imide groups, a reactive oligomer containing imide groups shows better compatibility with the epoxy resin. Although phase-separated structures are observed in the cured epoxy blends containing these oligomers, the extent of the phase separation varies with the curing temperature. Compared with blends with reactive oligomers without imide groups, cured epoxy blends with imide-based reactive oligomers exhibit a smaller phase separation size. The glass transition temperature (<i>T</i>_g) of the cured epoxy blends depends on the precuring temperature; a lower precuring temperature results in an increased <i>T</i>_g when the blend is fully cured at the final curing temperature. Compared with the unmodified cured epoxy, the cured epoxy blends with reactive oligomers show a reduction in both flexural modulus and <i>T</i>_g. However, the decrease in <i>T</i>_g is less pronounced in the cured epoxy blends with the imide-based reactive oligomer than in the blends with the reactive oligomer without imide groups.</p>