The aim of this study is to investigate effect of cement slurry consistency toward friction pressure during oil/gas cementing operation. Completion of an oil/gas well has become more important because the reserve has become harder to find. The oil/gas company cannot afford to lose million dollars they spent when locating, drilling and recovering the oil from the Earth if they failed it. The safety, health and environment also have become more important issue, because any completion problem can lead to prolong operation and creating more hazard and risk. Cementing operation plays a very important role during completion because it creates a secure conduit to bring the precious oil/gas to the surface and a place to install completion jewelry. During cementing operation lost circulation can be one of the serious problems that arise. Circulation is said to be lost when the cement slurry pumped flows into one or more geological formations instead of returning up casing annulus. This is due to sum of hydrostatic pressure and friction pressure is exceeding fracture gradient. Method that commonly used to calculate friction from American Petroleum Institute (API) assumed the cement slurry will exhibit time independent nature. Cement slurry consistency was found to have significant effect to friction pressure. © Maxwell Scientific Organization, 2012.