Many peoples know that´s important to study Mathematics but we need several time to understand and using it especially for technicians. In this literature study we give the relation between Bessel function and frequency modulation teclmics in communication system. We have compare mathematics method ( using Bessel function ), practical method and measurement and we have the results as follows :For fm= 2 kHz and 8 = 10 kHz. Calculation Band Width with Bessel function= 32 kHz. Calculation band Width with Practical Method= 24 kHz. Measurement in laboratory= 32,1 kHz.