Abstract: Underbalanced drilling realizes the controlled flow of formation fluids into the well, thus minimizes the risks of formation damage, lost circulation and pipe sticking, and improves bit performance and increases ROP. This paper list some items that should be considered in underbalanced drilling, such as casing program, formation conditions, selection of drilling fluid systems, determination of underbalanced pressure, suitable well control and circulation system, and as well as BHA program and bit selection, etc. This technology is successfully applied in two wells in Shengli Oilfield, some valuable experiences are achieved and the oil production is remarkably increased.