A modified three-dimensional (3-D) numerical model is proposed to study heat conduction in nanoporous aerogel by simulating the representative ordered structures. We introduce size effects of the aerogel particle and the pore into the numerical model to improve prediction accuracy for the effective thermal conductivity of aerogel. The numerical model is examined by comparing with available experimental data and 2-D numerical results. The coupled heat transfer between the aerogel solid and gas phases is investigated, and the effect of gas pressure on prediction accuracy of the present model is also discussed. The results show that the present model can accurately predict the effective thermal conductivity of aerogel when the aerogel density is above 120 kg/m(3) and the gas pressure is in the range of 10(4)-10(5) Pa. (C) 2014 Elsevier Ltd. All rights reserved.