The present work aims at clarifying the mechanisms of fluid-elastic vibration of tube bundles in two-phase cross flow. The experiment is conducted using air-water two-phase flow under atmospheric pressure. The test section is a 1.02-m long transparent acrylic square duct with 128 mm x 128 mm cross section, which consists of 3 rod-rows with 5 rods in each row. The rods are 125-mm long aluminum rods with 22 mm in diameter (P/D = 1.45). The natural frequency of rod vibration is about 30 Hz. The result indicated a diversion of observed trend in vibration behavior depending on two-phase flow patterns either bubbly flow or churn flow. Specifically, in churn flow, the fluid-elastic vibration has been observed to occur when the frequency in void fraction fluctuation approached to the natural frequency of the rods, but this was not the case in the fluid-elastic vibration in bubbly flow. This fact suggests the existence of mechanisms closely coupled with two-phase flow structures depending on the flow patterns, that is, static two-phase character-controlled mechanism in bubbly flow and dynamic character-controlled in churn flow. (C) 2004 Elsevier Inc. All rights reserved.