The effects of gultaraldehyde (GA) crosslinking, basic chromium sulfate (Cr) tanning, and thermal treatments on the loop strength of spun collagen fibers were studied. The loop test have been carried out using a piano wire as a model loop. The promotion of GA crosslinking led a significant decrease of the strength and elongation at break. On the other hand, the promotion of Cr tanning gave no change. These properties decreased with an increasing shear stress at bending region of loop. The improvement of these mechanical properties were realized by thermal treatments of both GA crosslinked and Cr tanned fibers. The thermal treatment temperature showed the maximum effect at 100 degrees C. However, there was no effect of thermal treatment for the noncrosslinked fibers. The fracture morphology of Cr tanned fibers is characterized by longitudinal splitting along the fiber axis in the bending region, although the fracture morphology of noncrosslinked and GA crosslinked fibers can be characterized by transverse fracture.