[en] Carbon nanotubes have been synthesised with high density on a large area of platinum pre patterned layer of silicon nitride solid substrate by means of chemical vapour deposition of acetylene at 500 ⁰C in ammonia ambient. The effect of the microwave induced thermal annealing, in atmospheric air, on the morphology and electrical transport properties of nanotubes films has been evaluated. At increasing annealing time the thermal treatments leads to a progressive reduction of the nanotubes' mean diameter and to a dramatic change in the electrical conductivity of the film. Such modifications have been directly attributed to a strong reduction of the amorphous carbon amount as a consequence of the thermal annealing by microwave