Recently, for the applications such as modern communications, synthetic aperture radars (SAR), electronic detection, electronic counterplot, and radar polarmeters, etc., the dual-polarization and polarization-agile functions of antennas are demanded and their optimization designs of systems are required. Their theory and further application are being in the process of developing. In this dissertation, the dual-polarized microstrip antenna elements and arrays of square patches are investigated systematically and experimentally and the proposed polarization-agile active microstrip antennas of amplifier type is realized and studied. Four subjects are included.Firstly, the analytical expressions for the input and mutual impedance of three dual-polarized square patch antennas fed with different manners are derived. The characteristics of the input impedance, VSWR (voltage standing wave ratio) and the isolation are analyzed. The effects of the design parameters on the characteristics of the antenna are studied with the results suitable for engineering applications. These elements are tended to form array structures and be integrated with active circuits.Secondly, a set of effective formulas for the cross-polarization radiation patterns of the dual-polarized microstrip array is obtained. The theoretical and experimental results of 16-element arrays are in good agreement.Thirdly, the FDTD method is applied to study the effects ofstructure parameters of the antenna element, such as the relative dielectric constant and thickness of the substrate, the width of the feed line, on the element characteristics including the resonant frequency, reflection loss and isolation. Further more, the transmission coefficient from one element to the next one and the mutual coupling between two elements are investigated .Finally, a series of experimental studies is made on the proposed polarization-agile active microstrip antenna witha low noise amplifier (LNA). The polarization-agile of the whole array from linear polarization to circular one is achieved by electrically controlling the only active circuit at the feed port. The practical active circuits for the polarization-agile and amplification are presented. The phase shifter and LNA are optimized systematically by the TOUCHSTONE package. As the passive array is interconnected with the LNA, the active gain is enhanced and the VSWR as well as the isolation characteristics are improved. Only one set of shifter and LNA is used in this array, but the functions and applications of the array are expanded significently, which not only can receive various polarized waves but can be size-minimized to meet the given gain requirement as well.