Abstract: The space-wave method, suitable for extended surface targets, is a preferable method for permittivity measurement of ground materials at microwave frequencies. The inversion of permittivity from the measured data, which is established on using the Fresnel reflection coefficients, is easy if the measurement includes phase information. However, if the measured quantities are just the amplitudes of the reflected waves, the inversion will be much complicated by solving a quadratic nonlinear equation set, and so far only numerical iterative approaches have been reported in the literature. Unfortunately, the iterative process is highly unstable because of the extremely sensitivity of the inverse algorithms to the measured data and even to the iterative initial values. In the present work, analytical inverse formulations are presented which will give a unique inverse result if it does exist for a set of measured data. Error sensitivity of the inversion to the measurement errors are performed, as well.