Thermal conductivities of aligned carbon nanotube (CNT)-polymcr nano-composites were estimated using the off-lattice Monte Carlo simulation. High thermal conductivity to density ratio is theoretically and experimentally recognized as one of the exceptional properties of CNTs. Aligned CNTs combined with existing advanced composites are being explored for macro-scale aerospace structures that benefit from thermal tailoring and light weight. Accurate thermal transport models within different polymer nanocomposites, and larger-scale and complexity composites, remain to be developed. The model previously developed for single-walled nanotube (SWNT)-polymer composites was modified to simulate the thermal property of aligned multi-walled nanotube (MWNT)-polymer nanocomposites of different volume fraction. Random walk simulations of thermal walkers are used to determine the interfacial resistance to heat flow inside the nano-composites in the directions parallel and perpendicular to the CNT alignment axis. The thermal equilibrium factor between the MWNTs and the composite matrix material is also determined numerically in this study. The CNT-polymer samples were fabricated for thermal conductivity measurements using two methods: the pump-and-probe method and the infrared microscopy. Aligned SWNT and MWNT forests were grown using chemical vapor deposition (CVD). The MWNTs were mechanically densified up to ∼20 % aligned-CNT volume fraction. The MWNT forests were immersed in an aerospace-grade thermoset resin, and cured. Near future work is to compare the simulated effective thermal conductivities of the CNT-epoxy composites with the measured data of the fabricated specimens to determine thermal boundary resistance between CNTs and the polymer.