Interparticle collision is an important phenomenon in the mechanics of two-phase flow. Most past researches on this problem have been based on the analogy with kinetic theory of gases with few experimental validations of the results. While this paper presents the directly measured experimental data of collision rate in vertical two-phase flow. This research used a high-speed camera and Particle Tracking Velocimetry (PTV) algorithms to focus on the interparticle collisions, especially the collision rate, on a millisecond time scale and a millimeter space scale, which is the particle size (1.8 mm in diameter). The camera speed was normally 2000 frames per second with an exposure of 1/2000 s and a laser power of 25 W for particle velocity of less than 4m/s in the collision region and particle fractions of about 0.015. A manual count of the collision numbers was chosen as the most accurate method for the determination of the collision rate. The PTV algorithms were applied to calculate the particle number density and relative velocities. The correlation between the particle collision rate, the particle number density, the average relative velocity between particles and the measured granular temperature, was compared with commonly used relations based on kinetic theory. The results demonstrated that great differences exist between the theoretical and experimental results for these experimental conditions. The collision rate determined experimentally is much lower than the theoretical estimation based on kinetic theory. This means the theoretical correlations overestimate the collision rate in the gas-particle flows. This discrepancy may be attributed to the assumptions in the theoretical collision model and experimental error. (C) 2004 Elsevier Ltd. All rights reserved.