We have synthesized a mat of silicon nanowires (approx. 10 mg in weight and 20 cm(2) in size) via the zinc reduction of silicon tetrachloride. The mat consisted of smooth and long nanowires with a median diameter (d(50)) of 40 nm. The flow pattern of gas inside the reactor that was used to form the mat significantly affected the formation of nanowires, even though conditions such as temperature or flow rates were fixed. In the best flow pattern, nanowires preferentially formed on a fixed position in the reactor and the mat gradually thickened as the reaction time increased. Computational fluid dynamic (CFD) simulations revealed that both a slow circulation flow of gases and a reaction front formed around the best position in the reactor, which resulted in the largest yield. This suggested that the reaction gases required a longer residence time for the preferential formation of the nanowires. The mechanism of this selective nanowire formation is discussed.