Abstarct: Nowadays, monitoring instrument is of particular importance in controlling the stability and safety of structures, since the selection, type and placement of instruments in a structure can identify the phenomena and destructive factors such as cracks and displacement in the structure and pore pressure and reduce stress and etc. One of the monitoring instruments used to measure surface and depth displacements is magnetic/rod extensometer. These kind of instruments are placed inside the borehole that are drilled in the soil and rock, and the space inside the borehole is filled with bentonite-cement grout. In some cases, expectable measurements with the results of instrumentation data are different, and in most cases, this difference and error can be attributed to the lack of suitable connection of the attachment point and the type and properties of the grout to the surrounding environments. For this purpose, a series of laboratory tests with grout mix design were used to determine the mechanical properties and the interaction between the grout and ABS tubes. Moreover, to investigate the effect of control parameters on displacement measurements, a numerical study was performed using FLAC3D software with modeling the borehole extensometers. In this regard, the effect of the ratio of grout-soil elastic modulus, soil/grout interface parameters (friction angle and cohesion) has been investigated. The results of laboratory and numerical studies show that the higher the percentage of mixing of water to cement and bentonite to cement, the compressive strength, the stiffness of the sample and the shear stress of the interface will be reduced. The error percentage of the measurement of instrumentation has a direct correlation with the stiffness of the borehole filling grout, which decreases with the ratio of the surrounding soil stiffness to the stiffness of the grout. The higher is the stiffness of the grout than the surrounding environment stiffness, the lower is the amount of displacement transferred to the instrument and results in a higher error percentage. Also, for grout-soil/grout-rock elastic modulus ratios of more than 10% for three different soil types (soft, medium and stiff) and two types of weak and medium rock masses, the error is significant and the error percentage in the soft soil type including instrument is more than the others. Additionally, the effect of friction angle and interface cohesion on the measurement error was investigated. The results indicate a negligible effect on the friction angle and tangible effect of interface cohesion in the error percentage of measurement; so the more the interface cohesion is closer to the cohesion of the environment around the grout, the lower the error percentage is. Finally, according to the obtained results, a suggested pattern is presented for the suitable range of the filling grout stiffness.