A planar slot antenna is designed and fabricated for 10mm square 60-GHz-band ultra-small wireless personal area network (WPAN) modules using low-cost organic substrates. Effects of ground and connector are discussed using computer simulation. As for planar slot antenna, it is proved that multilayered large grounds structure is most effective for gain enhancement, however, influence of the connector is very little. From measurement results, relative bandwidth of over 26.3 %, radiation efficiency of 66% at 60-GHz, and maximum actual gain of 7.1 dBi at 60-GHz are obtained. Organic substrates have enough potential for being used as materials of substrates for low-cost 60-GHz-band planar antenna.