Abstract: In order to investigate the changing rule of dielectric constant,the dielectric constant of colza oil,blend oil,sunflower oil,olive oil,corn oil,sesame oil,soybean oil and peanut oil were studied by the open-ended coaxial probe technology at 25℃. And the compositions of the fatty acid were measured. The change of acid value and peroxide value for soybean oil were been measured during frying,and connected to the changing rule of dielectric constant. The results indicated that at lower frequencies( 100 ~ 180MHz),the positive relationship was between dielectric constant and unsaturated fatty acid except for peanut oil; at higher frequencies( 300 ~ 10000MHz),dielectric constant of soybean oil was increasing with the content of linoleic acid.The acid value of soybean oil was increasing with the frying time,but peroxide value was increasing and then decreasing during frying.The dielectric constant was increasing during frying.At the same frequency,the positive relationship between dielectric constant and quality variation were found in this study.