After the exploration and identifying the location of the reservoir, the drilling operation is implemented for the production of oil and gas. The process of the drilling operations has a significant role in the petroleum industry and it can be one of the most expensive activities. Mud lost is the penetration of drilling mud in permeable formation at each depth partially or completely. These formations which are called thief zone are one of the common reasons of increasing non-productive time (NPT). Drilling mud does a lot of tasks and one of them which are more influenced by fluid lost is hydrostatic pressure maintenance that prevents of gaining fluid into the well. A correct handling of preventing or decreasing mud lost can reduce the problems caused by fluid lost. Experiences showed that preventing mud lost is cheaper and more effective than treatment. Usually in the phenomenon of mud lost, the fluid is lost because of induced fractures much more than other reasons. Because mud pressure is more than formation fracturing pressure, these induced fractures are created.