In order to inject nanoparticles into cotton fibers, cavities in the fibers should be expanded. To expand the cavities, cotton fibers were immersed in 24 wt%NaOH aqueous solution for 1 h, and then dried at 100 degrees C for 4 h. NaOH remaining in the fibers was then removed by washing. SEM revealed many cavities of about 500 nm in diameter in the cross sections of the cotton fibers. The cotton fibers were immersed in aqueous solution of 30 ppm silver nanoparticles, and the solution was maintained in an atmosphere of supercritical CO2 at 20 MPa and 80 degrees C for 1 h. Many silver nanoparticles of 10-30 nm in diameter were observed in the cross sections by TEM. The cotton fibers with injected silver nanoparticles were washed with a magnetic stirrer for 3 h, after which the number densities of particles in the fibers were almost the same as before the washing, showing that the particles had not been washed out. In an antibacterial test on the cotton fibers with injected silver nanoparticles, the value of antibacterial activities after 6 h of washing was 5.7, showing that the antibacterial performance had hardly decreased.