Morphological structures of epoxy resin toughened by a fluorinated polyimide were observed by SEM. Three layers formed spontaneously parallel to the film surface in the range of polyimide weight fraction w = 0.14-0.20, when the samples were cured at 130 degrees C. The two outer layers consisted mostly of the epoxy-rich phase. The middle layer consisted of bicontinuous domains of polyimide- and epoxy-rich phases with a characteristic domain size much smaller than the outer layer thickness. At w= 0.10-0.13, aggregates of polyimide-rich particles formed. Observation of the morphological development at w = 0.15 suggested that polyimide-rich domains were distributed over the whole sample in the early period.