High-average-power fiber lasers are interesting for many applications, including scientific instrumentation, industrial material processing, imaging, medical and military technologies. Fiber concept has well established them as a very attractive beam delivery system and gain medium for laser applications. Rareearth- doped fibers are one of the most promising solid-state laser concepts for efficient diode-pumped high power continuous wave laser systems. In fiber laser systems, the new generation fiber optical cables are used. Fiber laser systems are preferred due to the ease of usage in the field, high beam quality, low cost and low thermo-optic problems. Fiber laser system has an active medium where the light is amplified all along the fiber. The long active medium leads to reduced heat problems, high absorption rate of the light, high quality beam and less need for optimization of the cavity. We designed and developed a high power fiber laser system for metal processing. Material drilling and cutting, which are significant processes in industry, are performed with the powerful fiber laser systems. The main purpose of this thesis is to develop a laser system which enables fast and accurate cutting process with impressive properties such as compactness, low maintenance problems and so on. The fiber laser resonator was first numerically studied by using software and then various laser parameters and laser architectures were designed. In order to perform the numerical results, we designed a compact high power all-fiber laser cavity. In real world applications, the use of free space components to constitute the cavity leads to environmental instabilities and power loss, which results in a decrease of the fiber laser efficiency. One of the most important advantages of all fiber laser system is to handle environmental factors such as humidity, vibrations. To achieve high power level from the fiber laser systems, it is significant to use powerful pumping systems (high power diode lasers), passive laser cavity components (combiner, FBG) and also double-clad active fiber that operate at high power level. In this study, we used new generation laser cavity components. The pump sources have been analyzed in terms of power and wavelength stabilities. Also, experiments have been performed to keep pump sources at optimum temperature to obtain accurate wavelength to excite ytterbium atoms located in the active media. In order to constitute high power laser system, we integrated all fiber optic components for efficient operation. The laser has stable continuous wave regime in temporal domain due to the cavity design. The output power of the system is about 548 W. The M2 value of the system is less than < 1.7. The operation wavelength of the fiber laser was stabilized with FBG at 1.08 μm. We also demonstrate the capability of our fiber laser system for material processing. We successfully drilled and cut 1.7 mm thickness steel target with lower output power of fiber laser (100 W).