Kelvin Probe Force Microscopy is an attractive technique for characterizing the surface potential of various samples. The main advantage of this technique is its high spatial resolution together with high sensitivity. However as in any nanoscale measurements also in case of KFM it is extremly difficult to describe the uncertainty of the measurement. Moreover, a wide variety of measuring conditions, together with the complicated operation principle cause situation, where no standard calibration methods are available. In the paper we propose the model of the KFM microscope and analyze the uncertainty of the KFM measurement.