A large amount of parallel silicon nanowires, placed perpendicularly to a silicon substrate (silicon nanowire forests), has been contacted and assembled in order to fabricate legs of a thermoelectric generator. This paper reports the measurement of the main parameter for thermoelectric applications, which is the thermal conductivity. The reported value, which confirms the strong reduction of the thermal; conductivity in nanostructures, is measured on a large amount (10^7) of parallel nanowires with a  diameter variable in the range 60 - 120 nm, and takes into account eventual non uniformities which are unavoidable on surfaces of several mm^2. As silicon nanowire forests are very thin, it has been necessary to develop a suitable measurement  apparatus. The fabrication of devices based on silicon nanowire forests, the apparatus and the measurement procedure, as well as the the results, are illustrated and discussed.