Ghost fishing, caused by lost fishing lines and nets, has become a severe problem in marine environments. To eliminate ghost fishing in the ocean, the environmental degradation behavior of fishing lines must be understood. In this study, the environmental degradation of biodegradable nylon 4 fishing lines and commercial nylon 6, poly(ethylene terephthalate)(PET), and poly(vinylidene fluoride)(PVDF)fishing lines was simulated in the laboratory using an artificial weathering tester and biodegradation test in extracted seawater. To understand the degradation mechanism, the chemical and structural changes induced by photo-oxidation and biodegradation were investigated using tensile test, scanning electron microscopy, differential scanning calorimetry, gel permeation chromatography, infrared spectroscopy, and wide- and small-angle X-ray scattering. The results indicated that photo-oxidation occurred in the amorphous phase of the nylon 4, nylon 6, and PET fishing lines during ultraviolet (UV)exposure. The nylon 4 fishing lines exhibited excellent biodegradability, whereas the nylon 6, PET, and PVDF fishing lines could not be degraded by microorganisms in the extracted seawater. Both processes, i.e., photo-oxidation and biodegradation, were confined to the amorphous regions of nylon 4. Note that the PVDF fishing lines could not be degraded by UV exposure and biodegradation and, hence, should be recycled after use.