Spectral changes of ESR signals due to germanium associated Ge(3) centres were observed on heat-treated optical fibres and preforms. The defect centres essentially exist in as-drawn fibres, even if preforms have been oxidised prior to drawing. The 500°C heat treatment of Ge-doped fibres caused a remarkable decrease in the ESR intensity of Ge(3) centres, subsequently causing optical loss increase around the 1.39 μm wavelength.