The characteristics of high power laser beam (Nd: YAG, λ=1.06μm) transmission usinglow loss optical fibers are investigated. It is shown that the low loss plastic cladding qquartz glass core fiber can transmit even such high power as the 160W cw laser light and 120 kW (peak-value) pulse laser light without any damage. As an application of this high power laser transmission using optical fibers, semiconductor silicon processing has been performed, irradiating silicon substrates with the transmitted laser light. It has been found that the processed silicon surfaceis uniform and the processed spot has a sharp edge.