Commercial methods for producing multiwalled carbon nanotubes (MWCNTs), such as chemical vapor deposition (CVD), employ transition metal catalysts. Therefore raw MWCNTs usually contain metal catalysts and other carbon impurities. Acid treatments are commonly used to eliminate these impurities, which can reduce the length of the MWCNTs, thereby controlling their aspect ratio. These aspect ratio controlled MWCNTs can have many applications as electrode materials, biological imaging and sensing, etc. In this study, the aspect ratio of MWCNTs was controlled by an acid treatment with a 3:1 mixture of concentrated H2SO4/HNO3 for various treatment times. The results show that the acid treatment can control the aspect ratio of the MWCNTs. The aspect ratio controlled MWCNTs were observed by TEM, TGA and Raman spectroscopy.