Abstract：This paper took Guizhou’s main chili varieties‘Layan 102’and‘Sujiao No.5’as test materials；conducted field positioning experiments at 5 nitrogen application levels of 0，70，140，210 and 280 kg·hm-2 for 2 consecutive years．According to the relationship between above-ground dry mass and nitrogen concentration，the paper established a critical pepper nitrogen concentration dilution curve model and evaluated its feasibility in diagnosis of nitrogen nutrition in pepper．The results showed that the above-ground nitrogen concentration in pepper plants decreased gradually as the fertility process advances，the dry weight on the rise．A unified Nc curve model for 2 pepper varieties was established：Nc = 4.39PDM-0.28，R2 was 0.93．Model test result showed RMSE was 0.26 and n-RMSE was 10.08%．And there was a significant linear relationship between the simulated value and measured value，indicating model had a better stability．The nitrogen nutrition index（NNI）calculated based on Nc curve increased with the increase of nitrogen application quantity，and the variation range of NNI was 0.55-1.27 under different years，varieties，nitrogen application quantity and growth period．Even the correlation between NNI and relative yield（RY）was linear regression，reaching an extreme significant level．Comprehensive considering the pepper yield and NNI，210 kg·hm-2 was recommended as the suitable nitrogen application quantity for pepper in Guizhou region．
