Time variation on fading channels hinders accurate channel estimation in differential space–time modulation and deteriorates the performance. Decision-feedback differential detection is studied for block differential space–time modulation, and compared with conventional differential space–time modulation. It is observed that the proposed scheme does not suffer effective fading bandwidth expansion, as does the conventional scheme. An improved effective signal-to-noise ratio approach is proposed for analyzing the performance of the proposed scheme in time-varying flat Rayleigh fading. Theoretical analysis and simulations show the improved performance of the proposed scheme over the conventional scheme.