Background: Docosahexaenoic acid (DHA) is an essential omega-3 fatty acid for the human retina, skin, and cerebral cortex. Marine eukaryote Thraustochytriidae sp. was considered as a promising source for the n -3 LC-PUFAs production. However, the mechanism how the LC-PUFAs was synthesized in Thraustochytriidae sp. still remained unclarified. To explore the vital genes responsible for the DHA enrichment, the functional transcriptomic annotation was compared between the wild type and preeminent mutant of Thraustochytriidae sp. X2. Results: After the UV irradiation (50 W, 30 s), the mutant X2 showed enhanced lipid (78.88 % more) and DHA (23.77 % more) production compared with the wild type. Instead of EPA, 9.07 % of DPA was observed in the mutant X2. The comparative transcriptomic analysis showed that in both wild type and mutant strain, FAS was incomplete and lacked key desaturases, but genes related to the PKS pathway were observed. It was oberved that mRNA expression levels of CoA-transferase (CoAT) , acyltransferase (AT), enoyl reductase (ER) , dehydratase (DH) and methyltransferase (MT) down-regulated in wild type but up-regulated in mutant X2, corresponding to the increased intercellular DHA accumulation. Conclusion: These findings indicated the potential genes that can be exploited for high DHA yields in Thraustochytriidae sp..