Elemental analyzer/isotope ratio mass spectrometry (EA/IRMS) has been widely applied to analyze the N-15/N-14 isotope composition (delta N-15) of plants and soils, but the delta N-15 results may be inaccurate due to incomplete combustion of the high-C/N-ratio plant samples by EA. Therefore, it is necessary to develop a method to solve the problem of imperfect combustion. In this study, we used two methods: 1) adding copper oxide powder to the samples, and 2) increasing the O-2 flow (from 100 mL min(-1) to 200 mL min(-1)) for the auto sampler inlet purge line of the EA. The delta N-15 values of the plant samples became more positive and tended to be stable after complete combustion. Also, the required blank samples for each plant sample decreased with increasing amount of the added CuO powder. However, at 200 mL min(-1) of the oxygen flow in the EA, complete combustion could not be achieved without adding copper oxide, but this was done with decreased amount of CuO powder. Therefore, mixing cupric oxide into the high-C/N-ratio samples was an efficient, simple and convenient way to solve the problem of imperfect combustion in the EA.