Abstract: ObjectiveEffects of different nitrogen application rates on chloroplast ultrastructure and photosynthetic rate in leaves of sugarcane were investigated, cytological mechanism of nitrogen influencing sugarcane photosynthesis was also studied to provide reference for rational fertilization of sugarcane. MethodThree sugarcane cultivars, GT11, ROC22 and GXB9, were planted in tub in greenhouse. Three nitrogen levels(low nitrogen, medium nitrogen and high nitrogen) were set up, equally 0, 150 and 300 kg/ha urea were applied to the sugarcanes. Sugarcane leaves were sampled, and chlorophyll relative content(SPAD), leaf area, specific leaf weight, chloroplast ultrastructure were analyzed at the elonga-tion stage, and photosynthetic rate was determined. ResultSPAD and leaf area of the three sugarcane cultivars in-creased with the increase of nitrogen level. The specific leaf weight in GT11 and ROC22 decreased significantly with the increase of nitrogen level(P<0.05, the same below), and specific leaf weight of GXB9 was the lowest at medium nitro-gen level. The chloroplast ultrastructure in GT11 was normal at high nitrogen level. ROC22 had the largest chloroplast number at medium nitrogen level, but grana lamella number was significantly less than that at high nitrogen level. Chloro-plast number in GXB9 was the highest at medium nitrogen level, but grana lamella number showed no significant difference compared with that at high nitrogen level. Photosynthetic rate in GT11 and ROC22 increased with the increase of nitrogen application rate. Photosynthetic rate in GXB9 reached the maximum at medium nitrogen level. ConclusionPhotosyn-thetic rate of sugarcane is closely related to chloroplast number, chloroplast ultrastructure and grana lamella number, and these traits are affected by sugarcane varieties and nitrogen application rates.