The critical Rayleigh number Ra-C at the onset of natural convection was studied by linear stability analysis for high porosity anisotropic horizontal porous layers of uniformly arraying vertical thin circular wires stretched across a hot and a cold surface. Navier-Stokes equations including flow resistance by the wires were solved since Darcy's law cannot be applied due to high porosity. Ratio of permeability in the horizontal direction to the vertical direction is constant to be 0.5 but ratio of effective thermal diffusivity is changed dependent on wire materials. The critical Rayleigh number Ra-C for the case of xi = 1 obtained by the analysis agreed well with the experiment. (C) 2009 Elsevier Ltd. All rights reserved.