A nanostructured film electrode, a multi-wall carbon nanotubes (MWNT)-modified glassy carbon electrode (GCE), is described for the simultaneous determination of guanine and adenine. The properties of the MWNT-modified GCE were investigated by scanning electron microscopy (SEM) and cyclic voltammetry. The oxidation peak currents of guanine and adenine increased significantly at the MWNT-modified GCE in contrast to those at the bare GCE. The experimental parameters were optimized and a direct electrochemical method for the simultaneous determination of guanine and adenine was proposed. Using the MWNT-modified GCE, a sensitive and direct electrochemical technique for the measurement of native DNA was also developed, and the value of (G+C)/(A+T) of HCl-digested DNA was detected.