Carbon nanotubes (CNTs) with macroscopic length and controllable orientation are ideal building blocks in micro/nano-mechanics, electronics and super-strong materials.Towards their applications, it is important to realize the synthesis of high-areal-density horizontal ultralong CNT arrays.The areal density of ultralong CNTs is tightly connected with the quantity of active catalysts with optimized sizes, whereas catalyst nanoparticles on the smooth surface of silicon substrate tend to agglomerate into larger ones at high temperature.