We developed a simple method to control the length of carbon nanotube array by using oxygen plasma etching. In this way, we could obtain a carbon nanotube with a uniform length (20, 30, 50, $70;{mu}m$), that was parallel to the substrate. Moreover, our growing method of carbon nanotube array gives a uniform diameter ~3.5nm, which is consistent with our previous results. Using the same etching method, we demonstrated the carbon nanotube radio frequency identification (RFID) antenna. The results could be useful for carbon nanotube applications such as flexible and transparent conductive films.