In this paper, an advanced boundary element method (BEM) with thin-body capabilities is applied to model multiple cells of fiber-reinforced composites with the consideration of the interphases. Effects of the multiple-cell models, as compared to the unit-cell model. in determining the effective material constants in the transverse plane, are studied. In this BEM approach, the interphases are modeled as thin elastic layers based on the elasticity theory, as opposed to spring-like models in the previous BEM and some models based on the finite element method (FEM). The BEM approach to the multiple-cell modeling is compared with the FEM approach. The advantages and disadvantages of the BEM as compared with the FEM for the analysis of fiber-reinforced composites are discussed. It is shown that the developed BEM is very accurate and efficient in the modeling and analysis of fiber-reinforced composites, and that different cell models can have marked influences on the evaluations of the effective modulus of fiber-reinforced composites. (C) 2001 Elsevier Science B.V. All rights reserved.