In signal processing, there are mainly two types of filters exist and the are Finite Impulse Response(FIR) filter and Infinite Impulse Response(IIR) filter. Finite Impulse Response(FIR) filter can be designed form Infinite Impulse Response(IIR) filter by various techniques. The widely used technique is the window technique. In this paper, the designing analysis of FIR filter are shown by Blackman window and the Rectangular window. The output of the FIR design by Blackman window and the Rectangular window are shown in this paper by simulating the code in Matlab 7. The matlab program returns with a satisfactory result with proper magnitude plotting.