Extremely flexible hollow fibers with 100-μm-bore size or less were developed for infrared laser delivery. Fabrication process and transmission properties of the ultra-thin hollow fiber were discussed. The silver layer was inner-plated by using the conventional silver mirror-plating technique. Concerning the fabrication parameters used up to now for 320-μm bore-sized fibers, the target flowing rate for plating solutions was 10 ml/min. Parallel bundles of silica capillary were used to increase the cross-sectional area. To achieve the target, bundles with 560 pieces, 1200 pieces, and 9600 pieces were used for the capillary with inner diameters of 100-μm, 75-μm, and 50-μm, respectively. The loss for the 50-μm bore size, 10-cm length silver hollow fiber was 10 dB at the wavelength of 1 μm.