This trial is designed to show the work of FIR Low-pass Filter by Fast Fourier transform how Windows method for resolving the contract exceeds the requirements set frequency. The filter is tested on an input signal x (n) consisting of a sum of sinusoidal components at frequencies 440, 880, 1000, 2000 Hz. By setting the length of the design of Windows method that M = 256 and which allows the Fast Fourier. transform to be N = 512 samples. To get through the low frequency 600 Hz frequency that suits the needs of the experiment.