Abstract: The effects of oxidizer appended during testing high C/N ratio plant sample were studied by EA-MS. CuO and V₂O₅ were used to blend with plant power alone or all together during determined by EA-MS. The results showed that standard deviation was lower when oxidizer was appended, and the reason was ascribed that sample was oxidized completely. Oxidizer affected plant nitrogen content determined and instrument’s background obviously. Higher nitrogen content was obtained and instrument’s background fall down rapidly, which means that sample oxidized instantaneously and more correct results could be obtained. Different oxidizer appended had different effects to nitrogen stable isotopic composition. The optimal condition were CuO appended in plant sample (M（plant）∶M（oxidizer）=4∶1), in which condition credible δ¹⁵N data could be obtained and instrument’s background was constant during sample testing. The method is simple, effective and cheaper for determined stubborn samples.