salinity is one of the undesirable environmental factors which affects both growth and crop production. The stresses cause a wide range of reactions in plants including the change in gene expression and cellular metabolism and the change in growth rate and production. To study the activity of the antioxidant enzyme Ascorbate Peroxidase (APX) and the expression level of its encoding gene in three Sistan melon Landraces (Cucumis malo L.), a factorial experiment was conducted in completely randomized design with three replications under control and salinity conditions (250mM and 350mM of NaCl). The relative expression of APX was compared by Real time PCR and enzyme activity was assayed in three melon landraces of Ghandak, Sefidak and Sefidak khatdar. According to the results, at 250 mM salinity level, the CmAPX gene expression showed an increase in comparison to control in all the three landraces and this increase in expression was significant at 5% in Ghandak and Sefidak landraces. At 350 mM salinity level, CmAPX gene expression was decreased in all the three studied landraces. At this level of salinity, the expression of CmAPX gene in the Khatdar Sefidak landrace significantly decreased (at 5% level) in comparison to control. More increase in antioxidant activity of APX with stable expression of CmAPX gene in Ghandak landrace was detected that shows this landrace could tolerate against salt stress better than the others.  Decreased expression of this gene in Sefidak khatdar landrace with reduced enzymatic activity showed sensitivity of this landrace to salinity. We suggest that other genes related to antioxidant enzymes as well as their relationship with each others be examined in Sistan melon landraces.