Alkali-resistant glass fibers (ARG) which are designed to reinforce cementitious and other alkali containing materials are coated with special sizings to provide superior alkali resistance. To determine how the environment affects the fiber surface properties and in turn mechanical behavior, two kinds of ARG fibers with different percentage of zirconia, namely NEG and CemFIL were subjected to various treatments and single fiber tensile tests. This paper summarizes tensile strengths of these fibers using a modified bimodal Weibull two-parameter cumulative distribution function. We use the effective surface thickness factor κ(d) to reveal the influence of various surface treatments on the extrinsic failure. The fracture surface was examined by atomic force microscopy topography and phase analysis to study surface corrosion resistance and identify primary fracture-causing defects. Among the three types of aqueous environments evaluated, an alkaline solution is the most aggressive to the fiber surface. The effect of tensile strength variability on the statistical analysis is correlated with Griffith fracture predictions. The relation of maximum height roughness of ARG fibers to fracture strength follows very closely the formula predicted by the Griffith fracture criterion. The influence of the sizing is shown to affect both the population and size of flaws on the fiber surface by healing effects. It highlights the importance of the sizing on environmental resistance of the alkali-resistant glass fibers.