Onion oil is considered as an important medical constituent which effect against heart disease, common cold, diabetes, osteoporosis and osteocopic pain. As result for expensive price of onion oil, some relater dealer go to adulterate onion oil by adding vegetable oils. Thus this study aimed to find a suitable method for detection of onion oil adulteration. Ten mixtures of pure onion and corn oils were investigated to identify adulteration of pure onion oil by following phytochemical assay, physical and chemical properties, as well as the microbiological methods. Results of qualitative phytochemical analysis indicated difficulty and not conformed adulteration of all mixed samples of onion oil. physical and chemical properties didn't give clearly results. While measurement of the diameter inhibition zone by using some fungal and bacterial strains, as well as microbiological methods have good results to detection of adulteration of onion oil mixtures. This study suggested that microbiological methods is the best and preferred for detection and information adulteration of some commercial onion oils compared with phytochemical analysis and determination of physical and chemical properties.