Silver nano-particles were injected in cotton fibers by using supercritical carbon dioxide to modify cotton fibers. The cavities in cotton fibers were expanded to inject silver nano-particles. The cotton fibers were treated as follows. The cotton fibers were immersed in sodium hydroxide aqueous solution, and were dried. Then, sodium hydroxide remained in the cotton fibers was cleared by washing. Many cavities in about 500 nm diameter were observed in the cross sections of the cotton fibers by scanning electron microscopy. The cotton fibers treated by sodium hydroxide aqueous solution were immersed in 30 ppm silver nano-particle aqueous solution, and the solution was pressurized for 1 h by carbon dioxide at 80 °C and 20 MPa. Transmission electron microscopy revealed a lot of silver nano-particles in 10 to 30 nm diameter existed in the cross sections of the cotton fibers. The cotton fibers in which silver nano-particles were injected were washed for 3 h by a magnetic stirrer. The number densities of silver nano-particles in the cotton fibers were almost the same before and after the washing. An antibacterial test was carried out for the cotton fibers in which silver nano-particles were injected. The antibacterial performance of the cotton fibers was remained after it was washed for 6 h.