The tensile strength of commercial glass fibers is examined by single fiber tensile tests. The fibers are analysed as received from the manufacturer (sized) and after a heat treatment at 500 degrees C (unsized). Weibull plots of the two series are used for comparison of the strengths of the sized and unsized fibers. It is shown that large sample sizes (over 60 tests) are required to lead to a reliable two-parameter Weibull distribution. The experimental tests clearly indicated that the unsized fibers were weaker in the low strength range, but had similar strength in the high strength range. An investigation of the fracture surfaces in the SEM showed distinct differences in the fracture patterns for high and low strength fibers. Fracture mechanics were applied to estimate the original flaw size and relate the observed fracture mirror surface to the fiber strength. Based on the observation of surface flaws, a "healing" mechanism by the sizing is considered likely for this type of fiber and sizing, thereby effectively increasing the strength of the fiber in the presence of larger surface flaws. (C) 2005 Springer Science + Business Media, Inc.