In this paper, a high cross-polarization discrimination (XPD) dual-polarization slot antenna for indoor small-cell base station is proposed. The proposed antenna has a symmetric structure and differential feeding structure. A symmetric structure and differential feeding structure induce destructive interference of cross-polarization components in the far field in major planes. Therefore, the high XPD can be achieved. Furthermore, a very similar radiation pattern is observed not only between the major planes of each polarization, but also between the two polarizations. In measurement results, the antenna has a 10-dB reflection bandwidth of 180 MHz, port isolation between two polarization ports of more than 40 dB, a maximum gain of 4.2 dBi, and average XPD of 26.4 dB.