Incorporation of carbon nanotubes (CNTs) into such functional devices as transistors, sensors and field emitters requires a control over their placement on substrate surfaces. In this paper, horizontal arrays of multi-walled carbon nanotubes (MWNTs) have been assembled on silicon substrate surfaces using a gas flow method. The influences of the flow rate and the flow time on the alignment of the MWNTs have been systematically studied. Additionally, the field emission property of the horizontal MWNT arrays prepared using this method has been studied.