Continuous soil moisture assessment and evaluation is essential for irrigation management and planning. One of the common methods for measuring the moisture content of the soil is the use of a gypsum block. The use of gypsum blocks in some cases, including the presence of organic matter in soil, may cause an error in the measurement of moisture. For this purpose, a laboratory study was conducted to provide a calibration equation for gypsum block in silty clay soil with organic matter. This study was done in three treatments including different percentages organic matter in two replications in the Laboratory of Aburaihan Campus of Tehran University. In this experiment, after putting gypsum blocks in distilled water and applying the treatments, moisture changes and the blocks’ conductivity were measured at different times. The results showed that the gypsum block reading and the measured volumetric moisture content increased with increasing organic matter content. Also, for silty clay soil containing organic matter, the polynomial equation Y = 7.08X³-0.01X² + 0.65X-0.76 with a standard error of 3.09 and a correlation coefficient of 958.0 was obtained after fitting all the data.