Ultralong horizontally aligned carbon nanotubes (CNTs) have received extensive attention and shown great potential for applications in micro/nano devices, super-strong fibers, etc., owing to their perfect structures, extraordinary electrical, thermal and mechanical properties, Compared with agglomerated CNTs and vertically aligned CNTs, the growth windows of ultralong CNTs are very narrow and the areal densities of ultralong CNTs are usually very low.The reason why very low portion of carbon nanotubes (CNTs) nucleated in catalyst region on substrates can grow into ultralong ones during gas flow directed chemical vapor deposition (CVD) process was investigated.