Space-time block coding has recently been proposed as one of most promising transmission techniques over multiple-input multiple-output (MIMO) fading channels. In independent identically distributed Rayleigh fading channels, orthogonal space-time block codes are able to provide full diversity while requiring extremely simple encoding and decoding. However, keyhole effects in realistic MIMO fading channel severely degrade the performance of MIMO systems in terms of link quality as well as channel capacity. In this paper, performance of orthogonal space-time block codes in MIMO fading channels under keyhole condition is analyzed systematically. Achievable diversity gains of keyhole channels are derived as well as closed-form expressions for error rate of space-time block codes. Simulation results are also provided to verify our analytical results.