The output power of fiber lasers increased very fast during the past decades,more than kilowatt can be obtained from single fiber laser.In order to further increase the output power,the core diameter should be increased to avoid the damage of the core surface,but the beam quality will be decreased.In this paper,two methods are introduced:one is to design the large mode field fibers and another is increasing the core diameter with new technology to improve the beam quality.Some experimental results are given in detail.