Ultraviolet (UV) disinfection is an environmental-friendly technology for water treatment. However, design and operation of UV disinfection reactors is very difficult without a good model. In this work, motion trajectories of micro-organism particles in various UV water disinfection reactors with different shapes, sizes and lamp arrangements were simulated in the computational fluid dynamics (CFD) software: FLUENT, using the Monte-Carlo method and adopting a new methodology. The reactor performance based on micro-organism log reduction was analyzed under constant UV dosage. Effects of design and operating parameters on reactor performance were analyzed and discussed. (C) 2013 Elsevier B.V. All rights reserved.