Distributed feedback (DFB) fibre lasers usually have two longitudinal modes symmetrically located at each side of the Bragg wavelength because they have the same lowest gain threshold. In order to obtain single longitudinal output, π-phase-shifted DFB fibre lasers are often adopted. However, we found that, even with no π-phase-shift,the single frequency output can be acquired from Yb3+-doped fibre DFB lasers. The polarization beat frequency was measured to be 366MHz.