A novel compact design of planar multi-band slot antenna for IEEE 802.16m WiMAX system is proposed. With the use of C-and inverted L-shaped slots, multi resonant modes close to 2.6 / 3.5 / 5.5 GHz bands can be excited to meet the specifications of WiMAX system (IEEE 802.16m). The obtained impedance bandwidth across these operating bands can reach about 525 / 743 / 813 MHz for the 2.6 / 3.5 / 5.5 GHz bands, respectively. We derive more than 10% reduction in antenna size and gain compact operation only from 32 × 14 × 0.8 mm3 antenna dimension. The measured peak gains and total radiation efficiencies are about 3.0 / 4.3 / 5.2 dBi and 88 / 95 / 97 % for the 2.6 / 3.5 / 5.5 GHz bands, respectively, with nearly omni-directional pattern in the XY-plane.