This paper investigates the effects of channel estimation errors on error probability performance. A communication system for complex space-time orthogonal block codes is first formulated, allowing the generalization of further results. Based on an orthogonal pilot block, the resulting channel estimation error is shown to be white and Gaussian distributed, assuming a white Gaussian noise channel. Furthermore, we consider the SNR degradation, and obtain the lower BER bound and an accurate approximated BER given the BER under perfect channel knowledge condition. We also observe that such channel estimators are able to maintain the diversity order at high SNR.