Boron-rich nanowires with fivefold twinned cross-sections were synthesized by conventional chemical vapor deposition. Interestingly, it is found that the lengths of twin variants in some rhombohedral based nanowires were substantially unequal, implying differences in the growth velocities of twin variants along longitudinal directions. This phenomenon is in sharp contrast with the developments of face-centered cubic (FCC) based fivefold twinned nanowires. We suggest the high density of microtwins in subcrystals was responsible for such kinetic growth behavior. More importantly, the current study implies that twin defects serve as an indispensable factor in prompting the kinetic growth of fivefold twinned nanowires.