Ultraviolet (UV) disinfection is an environmentally friendly water treatment technology. Effects of different lamp arrangements on UV disinfection reactor performance have not been well-studied. In this work, the UV disinfection performance of reactors with various lamp arrangements was analyzed on a common basis. Computational fluid dynamics (CED) simulation software FLUENT was used to simulate microorganism particle motion in different UV water disinfection reactors. Applying the Monte-Carlo method, the reactor performance was assessed based on microorganism log reduction under constant UV dosage. Results for different lamp arrangements show that increasing number of lamps did not improve reactor performance despite more homogeneous UV fluence rate distribution. The lamp located directly below the water outlet retains particles inside the reactor for a longer time, thus enhancing reactor performance especially at low water flow rates. (C) 2014 Elsevier Ltd. All rights reserved.