As an appealing interference management approach, interference alignment (IA) has not been intensively studied in full-duplex (FD)-based multi-cells. In this letter, based on Bezout's theorem, we explore the feasibility conditions of IA in small cell networks, where base stations are equipped with FD hardware. The comparison of IA performance between FD and traditional half-duplex small cells in terms of the total degrees of freedom (DoFs) is also conducted. We find that the IA performance with FD is limited by the number of antennas at users. Furthermore, we verify that IA with opportunistic FD can be implemented to achieve higher total DoFs of the network.