Leucine (Leu) is an amino acid which is considered to be an important compound in health matters and lately is recognized as an indicator of carbon storage. For the first time, an interaction between the amino acid leucine and thermally denatured single stranded (ss) DNA has been demonstrated by applying voltammetry. As a result of interaction the characteristic peak of ssDNA, due to the oxidation of guanine residues, decreased. The interaction behaviour between leucine and ssDNA was studied with UV-vis spectrophotometry, the obtained results are in good agreement. The results of the interaction study were exploited in order to develop a SWV method for the determination of leucine at the ssDNA modified carbon paste electrode (CPE). Different parameters were tested to optimize the conditions of the determination. The peak of guanine was at around +0.86 V. Linearity was observed in the range of 1,4.10-9 - 3,5.10-8 mol/L( r= 0,9990 ) while LOD equals 4,9.10-10 mol/L. The method was applied in spiked soil sample and gave satisfactory results.