A dielectric loss peak with relaxation-type characteristic is observed in Bi5TiNbWO15 over 200-400℃. The modified Cole-Cole relation by introducing relaxation strength as another important fitting parameter is used to describe this temperature-dependent behaviour of dielectric relaxation process. This peak is considered to be associated with the oxygen vacancies inside the grains and with its activation energy by relaxation determined to be 0.76eV. The obtained broadening factor α is around 0.4, which indicates a strong correlation between those relaxation units. It is confirmed that the behaviour of this peak is due to the combined effects of the dielectric relaxation and electrical conduction by the thermal motion of oxygen vacancies. These results are further confirmed in Bi5TiNbWO15 samples through oxidization atmosphere treatment and Nd modification respectively.