In this paper, the added resistance of a large tanker is estimated experimentally and numerically in oblique sea. Experiments on ship motion response and added resistance in oblique sea are performed in the SSPA seakeeping basin. The experiments are conducted using the self-propulsion test for seven wave directions between 180° and 0°. In the self-propulsion test, the added resistance is estimated from the difference between the thrust of the propeller in calm water and waves. In the case of the head sea, the results are compared with those obtained from the captive test at the towing test of Seoul National University. As numerical method, two methods are selected: the strip method and the 3D Rankine panel method. The maximum value of the added resistance is observed between the incident wave directions of 180° and 150°. From 120°, the added resistance tends to decrease and the peak of the added resistance shifted to the short waves. Through the two numerical analysis methods, the tendency of added resistance and the cause of the change of the added resistance in the oblique sea are investigated.