Fracture mechanics was used to evaluate the strength and fracture characteristics of optical fibers. Optical fibers of single- and multi-mode were subjected to uniaxial tensile testing and two-point bending. The fracture strength data of both single- and multi-mode under either testing were found to be very similar. The fracture stresses at 50% fracture probability for tensile testing and two-point bending were measured to be 4.5 and 5.1 GPa, respectively. Our data on tensile strength versus bending strength is consistent with the Weibull statistical nature that a larger critical crack is more likely to be found as specimen size increases. The fracture characteristics of each tested optical fiber specimen were evaluated by using an SEM. A critical flaw on the surface of glass fiber was found to be the fracture origin for specimens under either tension or bending as expected. Using fracture mechanics, the critical fracture flaws of optical fibers were calculated to be in nanometer size. The stripped fibers were found to be much weaker than optical fibers. Fractographic examination revealed that glass fiber was susceptible to subcritical slow crack growth after polymer coating was removed. (C) 2002 Elsevier Science B.V. All rights reserved.