The wave height effect on the added resistance in waves is investigated by carrying out the resistance test in regular head waves with changing the incident wave heights for three full hull ships with different block coefficient from 0.81 to 0.87. Using the response amplitude of ship motions and the added resistance coefficient obtained in the regular wave tests, the short-term prediction is conducted and the prediction results are compared with the results obtained in irregular wave tests. As a result, the followings are obtained. Strictly, added resistance in regular waves is not proportional to square of incident wave amplitude, although the exponent should be two theoretically. (The exponent is called `exponent of wave height effect' in this paper.) Particularly, in the range of λ/ L ＜ 0.7 where λ/ L means the wave length-ship length ratio, the exponent of wave height effect is significantly smaller than 2.0. On the other hand, the exponent becomes close to 2.0 in the range of λ/ L ＞ 0.7. The averaged values of the exponent for three full hull ships are 1.77 for full load condition and 1.91 for the ballast load condition. The wave height effect of the regular waves for the response amplitude on the short-term prediction of heave and pitch is negligible. Generally, there is a tendency that the short-term prediction of the added resistance becomes larger when the regular wave test results with lower incident wave height are used. The prediction accuracy of the added resistance in irregular waves of a larger wave height case is improved by use of the regular wave test results with the corresponding larger wave height.