The effect of interfiber distance on the interfacial properties in three-dimensional multi-E-glass fiber/epoxy resin composites has been investigated using fragmentation test. In additions, the effect of the fiber surface treatment on the interfacial properties has been studied. The interfacial shear strength decreased with the decreasing the interfiber distance at the range of under 50 mu m and the extent of the decreasing was more serious as the increasing of the number of adjacent fiber. This is probably due to the fact that the interface between the fiber and the resin was damaged by the adjacent fiber breaks and the damage increased with closing the interfiber spacing and the number of adjacent fiber. It was found that the interfacial shear strengths saturated when the interfiber distance was over 50 tan, the ones were saturated regardless of fiber surface treatment and the ones were in close agreement with those of the single fiber fragmentation test. Finally, the interfacial shear strength evaluated using three-dimensional fragmentation tests are shown as real values in-site regardless of fiber surface treatment, interfiber distance and existing of matrix cracks. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1483-1490, 2010