To study the mechanism and functions of USM (Unsharp Masking) sharpening and to investigate its influence on image frequency component, 12 sine-wave images of different spatial frequencies are generated via a program. Then they are composed into a complete gray level image, which serves as original image for sharpening. The original image is sharpened with different parameters of USM. The original image and the sharpened images are processed using Fast Fourier Transformation (FFT) method with MATLAB software. After that, the corresponding Fourier spectral values of the 12 frequencies are extracted, the corresponding spectral value of original image is subtracted from the value of the sharpened images. The data results are finally analyzed and studied. Through the analysis of the spectrum, it indicates that the USM sharpening effect is stronger, while the USM amount and radius increase. But the effects to the spectral components are not balanced. If the USM radius is smaller but the amount is bigger, the effect to the medium and high frequency is stronger. The larger the amount of the USM sharpening and the USM radius are, the stronger the emphasis on the low and medium frequency components grows. The middle and higher frequency components are reduced, if the amount and the radius are all smaller.