High-performance artificial muscle is of great interest to robotics community. In this study, we investigate the influence of pre-stress on fishing line artificial muscle's tensile stroke. Fishing line artificial muscles with electrothermal actuation were made from nylon 6 mono-filament and resistance wires. The tensile stroke in time domain was found to exhibit different actuation behavior under different levels of pre-stress in spring-load test. The supplied current was kept constant through all the experimental tests to investigate the effect of prestress on the artificial muscle performance. The presented study provided new insight into implementation of fishing line artificial muscle towards robotic applications, especially regarding how to choose appropriate pre-stress for muscle integration.