Abstract: This paper proposes a decoding scheme in space-time block code system with three transmitting antennas and one receiving antenna for frequency-selective fading channels, which does not require channel estimation at the transmitter or receiver. By applying the subspace method to space-time codes, the direct decoding is accomplished from both a signal processing and space-time coding perspective. With the special orthogonal design of space-time block codes, the input of the space-time block code system is obtained from the subspace. The direct decoding is addressed for frequency-selective fading channels from a single carrier perspective. Monte Carlo simulations show the performance of direct decoding and the performance com-parison with the decoder using the perfect channel information.