This paper focused on investigating the longitudinal structure of kapok fiber through the observation of residual deformation and fracture morphology of kapok fibers. By analyzing the surface morphology of kapok fibers before and after different treatments, we found that a kapok fiber is essentially constituted with 5-10μm wide strips that spiral along the fiber axis to form a tubular structure. The spiral seams appear to be the weak locations of the fiber and often the fracture or bending points when suffering a large force or repeated small forces.