The Fe-Ni nano-polycrystalline thin films of 21 nm and 235 nm in thickness are prepared by molecular beam vapor deposition in the absence and the presence of a magnetic field as high as 6 T. The results show that in the absence of the magnetic field, the 21-nm-thick thin films are formed by the grain stacks, and the sizes of grains are about 6-7 nm. In the presence of 6 T, the 5-nm-thick flat layers of interconnected grains of 21-nm-thick thin films are first formed on the surfaces of the substrates, and the grains are then elongated along the surfaces of substrates. Later on, the 21-nm-thick thin films are formed by 6-7 nm-size-grain stacks. In the absence of the magnetic field, the average grain size of the 235-nm-thick thin film is 3.6 nm in the early growth stage, and it is 5.6 nm in the middle growth stage. The growth way of thin film is akin to columnar growth in the final growth stage, and the grains are elongated along the growth direction. In the presence of 6 T, the 5-nm-thick flat layers of interconnected grains of 235-nm-thick thin films are also formed on the surfaces of the substrates, and the grains are elongated along the surfaces of substrates. Later on, the 235-nm-thick thin films are formed by about 6.1-nm-size-grain stacks. Accordingly, the coercive forces in the out-of-plane and in the in-plane of thin films of different thickness values decrease by the 6 T magnetic field.