A frequency agile quasi-Yagi dipole sub-array is proposed as an array element for base station antennas. The sub-array consists of four printed Yagi dipole antennas loaded with PIN diodes. The lengths of the director and driven dipole elements can be changed by controlling the PIN diodes states so that the sub-array can operate in either 2.5 or 3.5GHz band. Simulation results show that by using a broadband microstrip-to-coplanar stripline (CPS) feeding structure, the sub-array is well matched with impedance bandwidth (|S11| ≤ 10dB) of 6.5% and 10.5% in the lower and higher bands, respectively. The end-fire fan-beam pattern is obtained across the dual bands, which makes the antenna element suitable for base station applications that require frequency reconfigurability. Predicted gains are 9.4 and 12dBi at 2.5 and 3.5GHz, respectively.