Abstract: It is well known that relay is able to neutralize some interferences in destinations. In this paper we use not only relay to neutralize interferences but also interference alignment to reduce the number of antennas needed in destinations. To this end, a scheme of cognitive relay-aided interference neutralization and alignment is proposed. To neutralize and align interferences, the relay needs to retransmit the signals using proper transmitting vectors. We demonstrate that the 3-user MIMO interference channels with cognitive relay can achieve 2M degrees of freedom (DoF) when each node has M antennas. It is a big improvement compared to k-user system using interference alignment which is able to obtain 3M/2 DoF. The transmitting vectors in sources and relay are carefully designed to not only satisfy the neutralization and alignment constraints but also achieve higher sum-rate, which can be proved by simulation results.