The present invention relates to a method for isolating and purifying only a certain unsaturated fatty acid in a high purity from fatty acids present in oils including vegetable oils and fish oils by means of crystallization. More particularly, the present invention relates to a method for isolating and purifying only the desired unsaturated fatty acid in a high purity from fatty acids present in oils by selectively using a urea-addition crystallization, and a cooling crystallization or a high liquid chromatography. Specifically, the present invention provides a method for isolating and purifying linoleic acid or oleic acid as unsaturated fatty acids, in a high purity of at least 99% by subjecting fatty acids derived from oils, particularly, a vegetable oil containing linoleic acid or oleic acid at a high concentration, such as safflower oil, corn germ oil or olive oil, as the raw material to two-step urea-addition crystallization using methanol and urea and then crystallizing the concentrated unsaturated fatty acid from an organic solvent under cooling at temperature of −5° C. to −10° C. without stirring, or a method for isolating eicosapentaenoic acid (EPA) as unsaturated fatty acid, in a high purity of at least 99% by subjecting fatty acids derived from oils, particularly, a fish oil containing EPA at a high concentration, such as sardine oil, as the raw material to two-step urea-addition crystallization using methanol and urea to obtain a concentrated unsaturated fatty acid having a high purity and then further purifying the high-purified, concentrated fatty acid by means of a high liquid chromatography using a column filled with Ag-silica or Ag-alumina.