A new model for calculating the coefficients of thermal expansion, CTE, in all three coordinate directions is developed for composites containing non-axisymmetric, ellipsoidal particles, i.e., three-dimensional ellipsoids (a(1) not equal a(2) not equal a(3)) characterized by two aspect ratios, rho(alpha) = a(1)/a(3) and rho(beta) = a(1)/a(2). The model makes use of Eshelby's equivalent tensor for the particles and utilizes the methodology from recent papers by Lee et al. The effects of the primary, rho(alpha), and secondary, rho(beta), aspect ratios and filler volume fraction (0-6%) on the various CTE for nanocomposites containing aligned inclusions are demonstrated by calculations for a matrix of nylon 6 and a filler of montmorillonite single platelets. The CTE in the longitudinal direction, alpha(11), decreases, as both aspect ratios increase. The CTE in the transverse direction, alpha(22), decreases as rho(alpha) increases but increases as rho(beta) increases. The CTE in the normal direction, alpha(33), is greater than that of the matrix and increases as rho(alpha) increases but decreases as rho(beta) increases. (C) 2008 Elsevier Ltd. All rights reserved.