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It has been shown recently that multiple-input multiple-output (MIMO) fading channels could experience a keyhole phenomenon, under which the performance of MIMO systems will be degraded in terms of link quality as well as capacity. In this paper, the performance of orthogonal space-time block codes in MIMO fading channels under keyhole condition is analyzed. Closed-form expressions for the error probability of space-time block codes as well as diversity gains of keyhole channels are derived. We prove that the maximum spatial diversity gain of a keyhole channel with to transmit and n receive antennas is min(m, n) when m ne n. In the case of m = n, the achievable diversity gain is less than n but higher than n - 1. Accordingly, for space-time block codes in keyhole channels, there are two forms of error rate expressions for m ne n and m = n, respectively. Furthermore, coding gains of various space-time block codes in keyhole channels are obtained. Simulation results are also provided to demonstrate the accuracy of our analytical results.