Methyl linolenate has been thermally induced in glass ampoules at 225, 250 and 275°C to isomerize into cis, trans isomers of linolenic acid. The isomer mixtures were separated by gas chromatography using a 120 m capillary column. All the 8 isomers of linolenic acid, 9c12c15c, 9c12c15t, 9c12t15c, 9t12t15c, 9t12c15t, 9c12t15t and 9t12t15t of the linolenic acid were observed in the heated mixtures. The formation of isomers followed a distinct pattern in all the heated samples. The trans bond formation at C-9 or C-15 seems to take place first in the molecules leading to the formation of 9t12c15c and 9c12c15t isomers. The formation of 9c12t15c isomer seem to be high energetic compared to 9t12c15c and 9c12c15t formation and appear at a later stage in the thermal induction. This difficulty in the formation of trans configuration also explains the difficulties in the formation of 9t12t15c and 9c12t15t isomers during thermal induction. Their concentrations are smaller than the 9t12c15t isomer in the prolonged heated mixtures.