An experimental study of the mechanical reliability of microstructured optical fibers (MOFs) is reported. Tensile tests were carried on five types of MOFs and two reference fibers, and the tensile strengths were analyzed using Weibull statistics. Optical microscopy of the surfaces of rupture allowed identifying the critical flaws and determining the failure mechanisms. First, it appears that the MOFs have lower tensile strengths than standard optical fibers. Second, the mechanical strength of MOFs was found to be related to the dimensions and morphology of the microstructure. Finally, fractographic examinations confirmed that MOFs can fail from defects located in the vicinity of the air holes, in contrast to standard optical fibers for which cracks always start propagating from defects located on the outer silica surface.