In this paper, the resistance increases of the ship in irregular waves are dealt with. The resistance increases of the merchant type hulls in irregular waves are obtained by the theoretical calculation According to the studies on these results, it is shown that the resistance increase coefficient is a function of the Froude's number, the non-dimensional mean wave period and the hull characteristics. If the Froude's number and the non-dimensional mean wave period are constant, the resistance increase coefficients of a hull are expressed by the maximum value of them, and the rate to the maximum value. Using these results, the simple estimating formulae for the resistance increase of the ship in irregular waves are obtained by the statistical analysis.