This book presents several designs of on-chip antenna and suitable for millimeterwave radio frequency wireless communications. On-chip microstrip patch antenna with different polarization senses and on-chip DRA has been designed using the conventional 0.18-mim CMOS process and used for wireless communications at 60 GHz. An efficient on-chip antenna is proposed by combining it with an artificial periodic structure, which can make the lossy silicon substrate equivalently act as a magnetic conductor. This artificial magnetic conductor plane replaces the normal perfect electric conductor plane and can reflect the incident wave in phase for a low profile antenna, resulting in enhancement of antenna gain performance. The antenna performance is more improved by employing a high resistive layer beneath the antenna and AMC plane. From the performance of the antenna, we can see great improvement in the gain, radiation efficiency and impedance matching bandwidth, which confirms the functionof the AMC structure and high resistive substrates without the use of any external off-chip components. Also an on-chip microstrip patch reflectarray is proposed for obtaining high gain on-chip antenna.