To investigate the difference in N isotope ratio (15N/14N, expressed as δ15N) among N sources (synthetic fertilizer, livestock manure, and manure compost), eight synthetic fertilizer, four livestock manure, and thirty-seven compost samples were collected and analyzed for δ15N. The mean δ15N values of N sources were -1.5 ± 0.5‰ (range: -3.9 to +0.5‰) for synthetic fertilizer, +6.3 ± 0.4‰ (+5.3 to +7.2‰) for manure, and +16.0 ± 0.4‰ (+9.3 to +20.9‰) for compost. The lower δ15N of synthetic fertilizer was attributed to its N source, atmospheric N2 of which δ15N is 0‰. Meanwhile, more 15N-enrichment of compost than manure was assumed to be resulted from N isotopic fractionation (faster loss of 14N-bearing compound than 15N) associated with N loss particularly via NH3 volatilization during composting. Therefore, our study shows that δ15N values could successfully serve in discriminating two major N sources (synthetic fertilizer and compost) in agricultural system.