In this paper, the aim is to solve the effect of a filter on an input signal by using fuzzy logic. We used the characteristic of lowpass filter for this application. If the input frequencies are very high, it will be reduced. We obtained a rule-base for emulating this characteristic, Two methods are used for this problem. In the first method, fuzzy inputs are signal values and the differences of the signal values. This is called "the difference method". The other approach is to calculate the average of two discrete values of the signal. As known, averaging is simple lowpass filter which smoothes out high frequency variations in a signal. But fuzzy approach is used for averaging of the signal. And this is called "the average method".