A tri-band bandpass filter design by using conventional stub loaded step impedance resonator (SLSIR) is presented. The proposed SLSIR consists of three tri-loaded open-circuited stubs parallel attached to a short and open-circuited stub. A tri-band bandpass filter is designed using two proposed symmetrical SLSIR's separated by a short-circuited stub. In this configuration, the filter produces three bands of frequencies at 0.9, 2.4 and 5.5 GHz, respectively. It is shown that the resonating frequencies can be independently controlled by loaded stubs. For better out of band rejection and increased selectivity, four finite frequency transmission zeros around three passband edges are achieved. The obtained fractional bandwidths are 23, 10 and 17% centred at 0.9, 2.4 and 5.5 GHz, respectively. The proposed design topology is fabricated on a substrate RT/Duroid 5880. The simulated and measured results show good accordance. The fabricated prototype has a compact size of (0.13 × 0.16)l g at 0.9 GHz.