We systematically investigated the relaxation time of Bloch oscillations (BOs) in GaAs/Al0.3Ga0.7As superlattices as functions of the bias electric field, temperature, and structural parameters using the time-resolved terahertz spectroscopy. It was found that the relaxation time dramatically increases by ∼80% and a clear Wannier–Stark ladder shows up when the bias field exceeds ∼9 kV/cm. The dominant dephasing mechanism of BOs was identified to be interface roughness scattering (alloy disorder scattering) below (above) the critical bias electric field. From the temperature dependence, it was also found that phonon scattering plays a rather minor role in the dephasing process. © 2003 American Institute of Physics.