Oil content is an important yield trait in Brassica juncea (L.) Coss. Improvements to yield levels of mustard by conventional breeding methods have reached a plateau. The application of transgenic technology is an area that has not yet been explored for improving the oil content of B. juncea. In this study, the effect of overexpression of AtDGAT1 (a key gene involved in oil biosynthesis) on the seed oil content of B. juncea was investigated. For seed-specific overexpression, the gene was linked to Arabidopsis thaliana oleosin promoter and mobilized into mustard through Agrobacterium-mediated transformation. Transformants were selected on MS medium containing 50 mg/L kanamycin, and a transformation frequency of 10.5% was obtained. A total of 10 transgenic events were generated. Analyses of seed weight, oil content, and other yield traits in T1 transgenics showed that seed-specific overexpression of AtDGAT1 significantly improved the oil content and seed weight. The maximum oil content increase observed in the transgenic seeds was 8.3% compared with the wild-type plants. Total fatty acid content was increased from 4% to 14% in six of the seven events. However, the content of oleic and linoleic acid was reduced and, of these two, oleic acid content showed drastic reduction.