The valence-electron structure (VES) of potassium niobate (KNbO3) in tetragonal phase was calculated based on the empirical electron theory (EET). The mixture of ionicity and covalency is embodied by the number of valence electrons. This property is very common for perovskite ferroelectrics. Using the VES obtained, the spontaneous polarization was investigated in the ferroelectric phase after the determination of the atomic effective valence electrons. The value of the spontaneous polarization is 31 Cm-2, which was compared with other theoretical and experimental values. All of above results demonstrate that the EET is an alternative method for studying the ferroelectric properties of materials, which is effective and simple. (C) 2008 Elsevier B.V. All rights reserved.