A novel multi-band design of planar slot antenna with compact operation 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 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 -plane.