Determination of soil water content in situ is complicated due to variations in dielectric constant of porous media. Time domain reflectometry (TDR) may meet the need for measurement of soil moisture with high accuracy. However, TDR-type instruments are expensive. A theory, which is called standing-wave ratio (SWR) of transmission line, is proposed in this paper. This theory has been proven a valuable tool for measuring soil water content. A method for calculating the characteristic impedance of a four-wire probe was developed. Furthermore, a prototype of a SWR-type sensor equipped with a new soil probe was designed.The measurement error was found to be within