Brushes made of carbon fibers are used to improve the thermal conductivities of phase change materials packed around heat transfer tubes. The transient thermal responses measured in brush/n-octadecane composites essentially improve as the volume fraction of the fibers and the brush diameter increase. However, there is a critical diameter above which further improvement is not expected due to thermal resistance between the fibers and the tube surface. A two-dimensional heat transfer model describing anisotropic heat flow in the composite is numerically solved. The calculated transient temperatures agree well with the experimental. A simple model is also developed to predict the heat exchange rate between the composite and the heat transfer fluid. The values of the correction factors are identified on the basis of the results for the anisotropic model.