Fluid-elastic forces acting on a square tube bundle (P/D = 1.42) in air-water two-phase cross flow were considered experimentally. A tube in the tube bundle was oscillated in the direction perpendicular as well as parallel to the two-phase flow, and fluid elastic forces acting on the oscillated tube as well as tubes surrounding the oscillated tube were measured. Measured fluid elastic forces were separated into coefficients of acceleration, velocity, and displacement of the oscillated tube, and the coefficients were nondimensionalized with two-phase mixture characteristics, which were obtained using the drift-flux model. It was found that the fluid elastic force coefficients could collapse very well as a function of void fraction and/or nondimensional gap velocity. These coefficients were used to estimate the critical gap velocity of fluid elastic vibration. The results agreed qualitatively with previous experimental results of Nakamura et al. (1986a,b).