The irregular wave forces on the piles of small diameters are experimentally investigated. The total in-line forces on single vertical pile and on each pile in array in irregular waves are measured. For single pile, the relationships between Cd, Cm and K-C number are determined with the best least-square method in time domain. In the case of two piles in the side-by-side arrangement and the tandem arrangement, a grouping-pile factor, KG, is used to consider the interference between two piles. Based on the experiments, the plots of KG versus K-C number and the relative distance, l/D, are given. The irregular wave forces on the piles of small diameters are experimentally investigated. The total in-line forces on single vertical pile and on each pile in array in irregular waves are measured. For single pile, the relationships between Cd, Cm and KC number are determined with the best least-square method in time domain. In the case of two piles in the side-by-side arrangement and the tandem arrangement, a grouping-pile factor, KG, is used to consider the interference between two piles. Based on the experiments, the plots of KG versus KC number and the relative distance, l / D, are given.