Abstract: Aramid 1414 fibers made in China and Kevlar49 fibers made in the USA as experimental materials were used in this paper. The differences of molecule structures were contrasted through degree of polymerization by the detections of infrared spectrum and viscosity. The Aramid 1414 fibers were put into distilled water and seawater made by ourself respectively, then using the supersonic-wave to deal with them. After this, using the SEM to observe, found that crack appears on the fiber surfaces, presented the fibrillation and plush-like microfine fibre. Meanwhile the strength weak link increased and breaking strength and elongation at break of fibers lowered. Molecular kinetic theory was used in this paper to study loss of fibers the mechanical property.