Virgin olive oil is a foodstuff rich in natural antioxidants (polyphenols and tocopherols) and presence of oleic acid at significant amount in olive oil (55-83%) makes it more resistant to the oxidation process while it is also responsible for the increasing of the plasma HDL and decreasing of LDL, thus it has potential health benefits and plays an important role in the prevention of the cardiovascular diseases. Dramatic increase in demand and price for olive oil has observed over the past few years that can be cause of adultration in virgin olive oil. Therefore authentication of purity is desirable. So far, several techniques have used to detect olive oil adultrations. In this study some physical, chemical and instrumental methods were used to detect the possibility of adultration in Iranian olive oils by mixing of other oils specially soybean, canola, sunflower and olive pomace oils that are common adultrants found in virgin olive oils. It was observed that among physical and chemical indexes, iodine value and refractive index in two olive oil samples (named as A and B) were significantly (p 0.01) higher than reference olive oil ( pure olive oil) and were above the official limits established for olive oil that were considered as a reason for high degree of unsaturation in these oilve oils. Further studies were carried out by using of instrumental methods including gas chromatography, nuclear magnetic resonance and differential scanning calorimetry. Fatty acid profiles of the samples A and B showed higher amounts of linolenic and linoleic acids (5.68,40.94% and 6.65,55.04% respectively) and significantly lower amounts of oleic acid. For this resaen, soybean oil was considered as a possible adultrant. Because of higher amount of linolenic and linoleic acids in samples A and B, intensity and integrated area of signals 2 and 7 were more than other olive oil samples while in other olive oils signal 2 was not observed and signal 7 had very low intensity. These results could be evidents of the proportions of linolenic acyl groups higher than maximum limit (about 1%) were observable in HNMR spectra. Crystallisation and melting curves in pure olive oil showed three peaks. The largest peak that was as main peak, could have been caused by the major group of triacylglyceroles such as OOL, POO and OOO. The results obtained from the that in samples A and B were due to the changing of FA and TAG profiles. Crystallisation and melting curves were simillar to those of soybean and sunflower oils. Also, because of increasing in unsaturation fatty acids, temperature of peaks, oneset and offset temperatures (T 0 , T f ) clearly siffted to lower values. Because of similarity between fatty acid profiles in pomace oilve oil and virgin olive oil, using of physical and chemical indexes (except non-saponifiable compounds) could not be useful for distinguishing between these two oils. On the other hands NMR spectra, cryst