- Voids are one of the most common types of manufacturing process induced defects in composite materials that have detrimental effect on the material properties. The void content can be reduced by carefully chosen process parameters, such as pressure and temperature, but often at the price of higher cost. A quantitative relationship between void characteristics and material properties would allow the trade-off between the cost and the desired product performance. The characteristics of interest include volume fraction, size, shape, and spatial distribution. In this paper, methods for determining effective elastic constants of unidirectional continuous fiber reinforced composites containing voids of various characteristics are presented. Finite element analysis (FEA) is performed on a representative volume cell based on observed void microstructure to determine the effective elastic constants. Analytical method based on Mori-Tanaka theory is also utilized for comparison. The predictions by FEA and the analytical method are compared with each other as well as with available experimental data. Overall good agreement is found. A parametric study reveals that the void content has severe impact on the out-of-plane modulus, while the in-plane properties are less significantly affected. For a given void content, the shape of the voids has different effect on different moduli. Flat voids are benign for in-plane moduli but undesirable for out-of-plane stiffness. Long voids reduce significantly the out-of-plane shear modulus, but have little effect on the in-plane properties. 2005 Elsevier Ltd. All rights reserved.