Mono-and periodically poled domain structures of ferroelectric KTiOPO 4 crystals have been studied by Kelvin probe force microscopy. This scanning probe method has allowed us to image the domain structure using a contact potential difference contrast between 180°ferroelectric domains. The measured contact potential difference between adjacent domains showed that the screening in the KTiOPO 4 crystals is provided by the highly mobile potassium ions. The screening charge density distribution and the resulting Debye length near the polar faces have been estimated.