The effect of interfiber distance on the interfacial properties in E-glass fiber/epoxy resin composites has been investigated using one and two fiber with various interfiber distance fragmentation test specimens. In addition, the effect of the fiber surface treatment on the interfacial properties has been studied. As a result, we found that the interfacial shear strength increased with the increasing of interfiber distances from 0 to 50 mu m and then the ones were saturated regardless of sized and unsized fibers. It was seen that the interfacial shear strengths saturated were in close agreement with those of the single fiber fragmentation tests. We also found that when the interfiber distance was very small, the stress distribution pattern was shown like one fiber, and when the interfiber distance was greater than 50 mu m, the stress distribution pattern was independent on between fibers. Finally, the interfacial shear strength evaluated using two E-glass fiber/epoxy resin fragmentation test method is shown as real values in-site regardless of the fiber surface treatment and interfiber distance. (C) 2007 Wiley Periodicals, Inc.