With the implementation of IMO Regulation on Minimum Propulsion Power and the revision of the ISO-15016 document concerning the sea trial speed correction method, the study of the added resistance in the waves that arises wide concerns has become one of the hot issues and the difficulties in the current study of the ship sea-keeping performance. Although the added resistance in the head sea conditions has been widely studied internationally, no substantial breakthrough has been made for the prediction of the added resistance in any wave direction, such as in the oblique sea and the following sea, yet. The methods for the numerical calculation and model experiment of the added resistance in any wave direction are firstly introduced. Then the numerical calculation and the model experiment of the added resistance in any wave direction are carried out for the 20 000 TEU very large containership. The numerical prediction method is verified and validated by comparison with the results calculated by the commercial software and the experimental results from Maritime Research Institute Netherlands (MARIN). It shows that the calculation results in the current study are more accurate than the results calculated by the commercial software, and the current model experiment method is feasible. The results can be used in the prediction of the added resistance of the ships in any wave direction, and thus the correction of the sea trial speed as well as the check of the regulation on the minimum propulsion power, which can provide good engineering application value.