Influence of side hulls arrangement for wave loads acting on cross deck of trimaran is investigated. We developed the prediction method for wave loads acting on cross deck of trimaran. The present method is based on strip method (STF method) and is not considering hydrodynamic interaction between main hull and side hull. And we carried out the experiment measured wave loads acting on cross deck of trimaran in head seas with side hulls position changes. Results show the vertical shearing force on cross deck increases and pitch connected moment decreases with shifting side hulls forward. And, it is confirmed that the present method can evaluate qualitatively influence of the side hull arrangement for wave loads acting on cross deck of trimaran in head seas.