The computer code MULTIFLEX was developed to take into account hydro-structural interactions in the hydraulic force calculation for structural integrity evaluation of a pressurized water reactor primary system during a transient induced by a loss-of-coolant accident. Code verification was performed by analyzing the fundamental phenomena pertinent to the mutual interaction between the hydraulic and the structural systems. Phenomena investigated were the virtual mass effect of reducing the in-air structural frequency to the in-water frequency, the fluid compression effect due to structural deformation, and the effective sonic velocity for hydraulic pressure wave propagation. Experiments emphasizing each aspect of the phenomena were selected and analyzed. In the analyses, the independent mass model for the structural dynamics was exclusively employed. Satisfactory agreement between the analyses and the experimental data was obtained.