The influence of gultaraldehyde (GA) crosslinking, basic chromium sulfate (Cr) tanning, and thermal treatments on the fracture behavior and morphology of spun collagen fibers has been studied. The fracture morphology of the fibers is characterized by longitudinal splitting along the fiber axis. Although the essential fracture morophology was not influenced by GA crosslinking, Cr tanning, and thermal treatments, the process of splitting depended on the kind of crosslink. Noncrosslinked and Cr-tanned fibers were split into fibrils, but GA-crosslinked fiber was split without fibrillation. The thermal treatments have two effects : One is decrease in number of defects and/or flaws; the other is gelatinization. In the thermal treatment above 140 degrees C, the gelatinization plays a more important role for the tensile properties than does the effect of a decrease of a defect. Gelatinization results in the enhancements of slippage and separation of macrofibrils and/or fibrils after the yield point.