<p>The present paper deals with a method of determining reciprocal relation between hull separation and draft on the resistance of a catamaran. The wave-making interference resistance is decreased as the hull separation grows larger, while the viscous resistance due to the increased weight of the deck is increased. From such reciprocal relation, the optimal hull separation is determined based on the wave-making interference resistance calculated by linear wave-making resistance theory with assumption that the individual hull of the catamaran is slender body, and on the change of draft according to the deck weight. The results show that the minimum value of the total resistance of the catamaran in the range of high speed appears at a point where the separation is smaller than 40% of the length of the catamaran. The proposed method can be applied to estimate the width of a catamaran in the conceptual design phase.</p>