Abstract: Buoyancy effect occurs when the drilling string is submerged in drilling fluid, and it is of great importance for drilling practice. It impacts the well trajectory control and causes influence on friction during pipe tripped. It should also be considered in many other field operations. This paper presents a new method for buoyancy calculation, based on detail mechanical analysis of pipe in vertical and inclined well. This method combines the calculations of drill collar, drill pipe and composite drilling string, and it takes the internal and external addition parts of drill pipe into consideration, which is usually ignored in conventional calculations.It is further testified and proved beneficial for field drilling.