Compounds released by mungbean roots potentially represent an enormous source of nitrogen (N) and carbon (C) in mungbean-oat intercropping systems. In this study, an in situ experiment was conducted using a 15N - 13C double stem-feeding method to measure N and C derived from the rhizodeposition (NdfR and CdfR) of mungbean and their transfer to oats in an intercropping system. Mungbean plants were sole cropped (S) or intercropped (I) with oat. The plants were labeled 5 weeks after planting and were harvested at the beginning of pod setting (Ip and Sp) and at maturity (Im and Sm). More than 60% and 50% of the applied 15N and 13C, respectively, were recovered in each treatment, with 15N and 13C being quite uniformly distributed in the different plant parts. NdfR represented 9.8% (Sp), 9.2% (Ip), 20.1% (Sm), and 21.2% (Im) of total mungbean plant N, whereas CdfR represented 13.3% (Sp), 42.0% (Ip), 15.4% (Sm), and 22.6% (Im) of total mungbean plant C. When considering the part of rhizodeposition transferred to associated oat, intercropping mungbean released more NdfR and CdfR than mungbean alone. About 53.4–83.2% of below-ground plant N (BGP-N) and 58.4–85.9% of BGP-C originated from NdfR and CdfR, respectively. The N in oats derived from mungbean increased from 7.6% at the pod setting stage to 9.7% at maturity, whereas the C in oats increased from 16.2% to 22.0%, respectively. Only a small percentage of rhizodeposition from mungbean was transferred to oats in the intercropping systems, with a large percentage remaining in the soil. This result indicates that mungbean rhizodeposition might contribute to higher N and C availability in the soil for subsequent crops.