Abstract: The Ultra-Wide Band (UWB) signal waveform distortions in Non-Line-of-Sight (NLOS) environment and its impact on localization accuracy are investigated in this study. The localization system can offer accurate position and ranging due to the fine time resolution of Impulse Radio (IR) signal. However, in completely NLOS environment, the IR-UWB signal will undergo deeply waveform distortion due to frequency dispersion, which cause inaccuracy in peak position detection in correlation receiver, consequently result in ranging error in time based localization system. Although, IR frequency dependence has already been addressed in channel modeling, in this study, the influences of material frequency properties and IR propagation distortion on completely NLOS localization system performance are taken into account through frequency domain method. Based on transient electromagnetic theory, the IR waveform distortions are compared in time domain using Inverse Fast Fourier Transform (IFFT). The NLOS ranging errors caused by both pulse distortion and geometric configuration are derived in a simple way. Numerical results shown that larger variations of material electrical parameters in UWB bandwidth lead to more severe waveform distortion and propagation loss in obstacles, which can considerably decline the ranging accuracy.