We show that PadB approximation can be effectively used for approximation of performance functions in discrete-event systems, The method is 1) obtaining the MacLaurin coefficients of the performance function and 2) finding a PadB approximant from the MacLaurin coefficients and use it to approximate the function, We use the method with the expected number of renewals in a random interval, GI/G/1 systems, and inventory systems. The results are very good.