Na0.5K0.5NbO3 (NKN) thin films were grown on Pt(111)/Ti/SiO2/Si substrates by using RFmagnetron sputtering. We investigated the behavior of the ferroelectric polarization. Well-saturated P-V hysteresis loops were observed, but the P-V hysteresis loops were sometimes distorted and consisted of three roughly parallel components, such as ferroelectric, dielectric and conductive components. Correction methods were proposed to identify the undesirable components and the pure ferroelectric polarization components from the observed P-V hysteresis loops. A new polarization hysteresis loop measurement technique, the double wave method (DWM), was applied to NKN thin films to obtain pure ferroelectric polarization and elucidate the behaviors of the polarization components.