Abstarct: Hydraulic fracturing method is the best known method for determining the magnitude and directions of the principle horizontal in situ rock stresses in the subsurface. Many investigations were conducted by researchers on the phenomenon as numerous parameters such as borehole diameter, pressure interval length and viscosity of fracturing fluid affect the results of the implementation of the hydraulic fracturing. However, there are vague hints about hydraulic fracturing method. As a result, more studies to determine the impact of various parameters on the process, are required. For this purpose, evaluation of the impact of distance between the packers on fracture pressure (breakdown pressure) in this procedure, was one of the subjects of this study. In this respect a laboratory physical model was designed and constructed and a significant number of tests were conducted. The results showed that by increasing the ratio of the length to the diameter of the interval the breakdown pressure decreases. Then, a new apparatus which allows laboratory Hydraulic fracturing experiments under true triaxial compression was developed at Shahrood University of Technology and laboratory hydraulic fracturing experiments were conducted to investigate the dependence of breakdown pressure upon two factors which could influence the use of the hydrofrac technique for in-situ stress determinations namely: the viscosity of fluid and borehole diameter. In other words, in this study, by using the new apparatus it was examined to see if the fracture pressure will change with the viscosity of fluids and borehole diameter. The setup of the system was in an order that specimens of 30 by 30 by 30 cm3 were able to be tested. It should be noted that, three steel rods of 30, 50 and 80 mm outer diameter were buried in the center of the concrete, during consolidation, to create simulated boreholes of different diameters in the specimens to examine the impact of borehole diameter on hydraulic fracturing breakdown pressure. Meanwhile, 3 types of Hydraulic oils of different viscosity (46H, 68H and 100H) were injected inside the boreholes to examine the effect of viscosity of fracturing fluid. Results show that breakdown pressure decreases with increasing borehole diameter and also there is an inverse relationship between breakdown pressure and viscosity of fracturing fluid. In other words, it decreases with increasing viscosity of fracturing fluid.