Natural nitrogen isotope composition (delta~(15)N) is an indicator of nitrogen sources and is useful in the investigation of nitrogen cycling in organisms and ecosystems. delta~(15)N is also used to study assimilation of inorganic nitrogen. However, the foliar delta~(15)N of intact plants, which is a consequence of nitrate assimilation occurring in the roots and shoots, is not suited for studying nitrate assimilation in cases where nitrate is the sole nitrogen source. In this study, Orychophragmus violaceus (Ov) and Brassica napus (Bn) plantlets, in which nitrate assimilation occurred in the leaves, were used to study the relationship between foliar delta~(15)N and nitrate assimilation. The plantlets were grown in vitro in culture media with different nitrate concentrations, and no root formation occurred for the plantlets during the multiplication stage. Nitrogen isotope fractionation occurred in both the Ov and the Bn plantlets under all treatments. Furthermore, the foliar nitrogen content of both the Ov and Bn plantlets increased with increasing nitrate concentration. Foliar nitrogen isotope fractionation was negatively correlated with foliar nitrogen content for both the Ov and Bn plantlets. Our results suggest that the foliar nitrogen isotope fractionation value could be employed to evaluate nitrate assimilation ability and leaf nitrate reductase activity. Moreover, high external nitrate concentrations could contribute to improved foliar nitrogen content and enhanced nitrate assimilation ability.