Added resistance due to waves is one of the major components for the evaluation of ship performance in actual seas, and it is important to be calculated accurately. Ocean waves are characterized as short crested irregular waves which consist of waves with various frequency and directional distribution. Added resistance in short crested irregular waves is usually evaluated by the superposition of directional wave spectrum and frequency response of added resistance in regular waves. Various evaluation method for added resistance in regular waves has been confirmed by tank tests so far. On the other hand, estimation method for added resistance in irregular waves has rarely been verified by tank tests since a given restoring force to a ship model affects to the long-period variation in irregular waves. In this paper, tank tests in long crested irregular waves for VLCC model and PCC model have been conducted, and added resistance in irregular waves has been measured and compared with the estimated results by the Newman's approximation. As a result, it has been shown that the measured long-period variation of added resistance in long-crested irregular waves can be expressed by the Newman's approximation and that the estimated results of averaged added resistance in long-crested irregular waves by the Newman's approximation with the consideration of wave height effect explained the experimental results better than that without the consideration of wave height effect.