The quality of medicinal cod-liver, edible and technical fish oils was evaluated on the basis of chemical and thermal analyses. The most substantial chemical variables, viz. density, refractive index as well as saponification, iodine and acid numbers were determined. The two groups of thermal variables were determined based on TG and DTG curves of the thermal decomposition of studied fish oils. The onset and end temperatures as well as successive mass losses were read from the TG curves, whereas the temperature range of peak, peak temperature, peak height and peak width at a half of peak height were read from the DTG curves. To find the relation between the chemical and the TG and DTG variables, regression and principal component (PCA) analyses were applied. Good linear relations were found between a majority of chemical and thermal variables. The results of PCA indicate that the TG and DTG techniques are very useful to determine the degree of deterioration of fish oil samples.