A single fiber fragmentation test is performed to investigate the effects of fiber surface treatment on fiber break behavior. Single glass fiber/epoxy composites with five different surface treatments are loaded in tension in the fiber direction. In-situ observation of the fiber fragmentation process is conducted by using an optical microscope equipped with a loading device. Fiber break density is measured as a function of applied strain. The microscopic failure around fiber breaks is also observed. It is seen that the fiber break behavior and the microscopic failure around fiber breaks are different with the fiber surface treatments. To interpret the experimental data, the authors' analysis of the stress transfer between the fiber and the matrix in a single fiber reinforced composite with fiber breaks is used. The analysis is based on a concentric cylinder model considering an interphase layer between the fiber and the matrix. By using the stress transfer analysis, fiber break density as a function of applied strain is predicted. By comparing between the experimental results and the analytical prediction, the validity of the analytical prediction is discussed.