Conjugated fatty acids (CFA) being novel type of biologically functional lipid substances, have received increased interest owing to their beneficial effects on human health. Among CFAs, conjugated linoleic acids (CLA) have been reviewed extensively for their occurrence, health benefits, industrial production and applications. On the contrary, it is worth noting that some plant seeds contain conjugated linolenic acids (CLN) at considerably higher levels (30 to 70 wt% lipid), although other kinds of CFAs including CLA are only found at concentrations less than 1% in natural products. It is thus imperative that CLN are the only CFAs that occur in higher quantities in nature and we have extensively screened CLN from different plant seed oils for their chemopreventive effects. In the ensuing review, we describe in detail the physiological functions of CLN isomers that occur in some plant seeds. CLN from natural origin showed growth inhibitory and apoptotic effects on cancer cells and the activity was dependent on their geometrical/positional structures. All trans CLN exerted stronger growth inhibition and more DNA fragmentation in human colon cancer cells than corresponding CLN isomer with cis configuration. CLN induced apoptosis through decrease of Bcl-2 protein. It will be interacted with increase in PPARγ signaling and up-regulation of gene expression of GADD45 and p53. Enhancement of PPARγ expression was also observed in the rats fed seed oil rich in CLN. Dietary feeding of these CLN-containing seed oil reduced the frequency of colonic aberrant crypt foci (ACF) in rat that is the precursor of colon carcinogenesis in short-term experiment and inhibited the development of colonic adenocarcinoma in long-term experiment.