As fish reef block hung in sea water is oscillated by rolling or pitching of derrick barge, theproblem is arising about the safety and accuracy of setting fish reef blocks. In this study, weprepared cubes and fish reef block models and were swung by string from a point which wasoscillated horizontally in still water. By measuring horizontal displacement of model, we calculated velocity and acceleration. Assumed that drag coefficient CD and added mass coefficientCA were not varied with a short period Δt elapsed, we obtained continuous values of CD and CA for each time from the solution of equation of oscillation.The values of CD and CA during oscillation varied widely with time elapsed and were different from the values obtained in steady flow. Therefore, equation of motion expressed well the oscillation of models by substituting CD and CA obtained from displacement of oscillation into this equation.