Outage performance of cognitive radios is analyzed in this paper. The scenario under consideration requires the cognitive radio to sense whether the primary user (PU) link is free (i. e. a spectrum hole exists) before making an active transmission using that link. Multiple antennas are available at the cognitive radio link to provide array gains at the sensing stage. We derive a closed-form expression of the outage probability for cognitive transmission by classifying it into several cases. A sensing threshold is deduced according to the PU arrival model illustrated in this paper. Simulation results verify our analysis.