This study was carried out to evaluate the biodegradability of viscose rayon and lyocell fibers, employing soil burial test, activated sludge test and enzymatic hydrolysis. Using X-ray analysis, crystallinity and morphology change was investigated. External changes after degradation were also observed by SEM and digital photographs. Vscose rayon fibers exhibited higher biodegadation than lyocell fibers, indicating that lower crystallinity favored the biodegradation. Among the biodegradability of lyocell fibers there was a tendency that fibers with lower crystallinity and higher moisture regain had higher values. When external changes after degradation being observed, it was shown that there were microorganisms growing on the surfaces of samples accompanying lading and weakening. From these results it was concluded that biodegradability of the specimens was most closely correlated to the moisture regain and crystallinity of fibers which reflects hydrophilicity and internal structure.