In recent years, there has been a tremendous renewal of interest in incorporating the N-fixing creeping legume, Mucuna bracteata, in the oil palm production system. Although a considerable amount of work has been done on M. bracteata, the N-fixing capacity of this legume has not been fully studied. Hence, an experiment using the 15N isotopic dilution technique was laid out to quantify the N fixed by M. bracteata in a 3-year-old oil palm plantation where P. conjucatum and mixed broad-leaf weeds were used as non-N-fixing reference plants. The results revealed that M. bracteata planted on Jawa Series soil derived 67%-84% of its N from N-fixation. The finding confirmed that M. Bracteata has the potential to contribute to the N pool of the oil palm cropping system, and has potential to be used as a green manure or even as an alternative N source to inorganic N. The high percentage of N fixed by M. bracteata also indicates that competition for applied N and soil N between M. bracteata and oil palm, if any, would be insignificant.