Irregularities are inherent to virtually all fibers, including the conventional textile fibers, the high-performance brittle fibers and newly developed nano-fibers. These irregularities can fall into two main categories: dimensional or geometrical irregularity (external) and structural irregularity (internal). For natural fibers such as wool, diameter variation along fiber length is a typical example of fiber dimensional or geometrical irregularity, while the presence of flaws and defects within a fiber signifies structural irregularity. The irregularities of fibers are bound to have major impact on the mechanical behavior of fibers. In recent years, there has been a growing awareness of the importance of this particular fiber attribute – fiber irregularity, particularly the within-fiber diameter variation. Instruments have been developed to accurately measure fiber diameter variations. In addition, with the development of computing technology, numerical modeling technique has been applied to examine the impact of fiber irregularity on fiber mechanical properties. This paper reviews research progress in the area of fiber geometrical irregularity and its effect on important fiber mechanical properties.