As a result of the necessity of aircraft engineering design and the progress of computational fluid dynamics (CFD), techniques of accurately predicting aero- dynamic drag are being increasingly explored. According to the momentum bal- ance, the drag can be represented by an integral over a cross-flow plane (called wake integration method) at an arbitrary distance behind the configuration. A for- mulation to reduce the size of the wake cross plane region required for calculating the drag is developed by using cutoff parameters of vorticity and entropy. This in- creases the calculation accuracy and decreases the computation time required. Numerical experiments are made to obtain the threshold values of these cutoff pa- rameters. The wake integration method is applied to predict drags of some exam- ples including airfoil, a variety of wings and wing-body combination. Numerical results are compared with those of traditional surface integration method, showing that the predicting drag values with the wake integration method are closer to the experimental data. The results also show that drag prediction within engineering accuracy is possible by using CFD and the numerical drag optimization of complex aircraft configurations is possible, too. As a result of the necessity of aircraft engineering design and the progress of computational fluid dynamics (CFD), techniques of accurately predicting aero-dynamic drag are being increasingly explored. According to the momentum bal- ance, the drag can be represented by an integral over a cross-flow plane (called wake integration method) at an arbitrary distance behind the configuration. A for-mulation to reduce the size of the wake cross plane region for for calculating the drag is developed by using cutoff parameters of vorticity and entropy. This in- creases the calculation accuracy and decreases the computation time required. Numerical experiments are made to obtain the threshold values ​​of these cutoff pa- rameters. The wake integration method is applied to predict drags of some exam- ples including airfoil, a variety of wings and wing-body combination. Numerical results are compared with those of traditional surface integration method, showing that the predicting drag value s with the wake integration method are closer to the experimental data. The results also show that drag prediction within engineering accuracy is possible by using CFD and the numerical drag optimization of complex aircraft configurations is possible, too.