The effects of low dose (10 9/cm 2) ion irradiation on the ferroelectric domains and on the ferroelectric↔paraelectric phase transition of triglycine sulfate (TGS) single crystals has been studied by using variable temperature dynamic contact electrostatic force microscopy. The evolution with temperature of ferroelectric domains of virgin and ion irradiated samples was observed in situ. In general, the phase transition of ion unirradiated samples is similar to that of virgin samples, which can be well described by Landau's mean field theory. However, on ion irradiated samples, there is a strong asymmetry of spontaneous polarization P for positive and negative domains. This asymmetry is due to the defects produced by ion implantation.