To investigate the effect of reaction temperature and nitrogen doping on the structure and fluorescence properties of carbon dots (CDs), six kinds of nitrogen-doped CDs (NCDs) were synthesized at reaction temperatures of 120, 140, 160, 180, 200 and 220 degrees C, separately, by using citric acid as carbon source and ammonia solution as nitrogen source. Nitrogen-free CDs (N-free CDs-180) was also prepared at 180 degrees C by using citric acid as the only carbon source for comparison. Results show that reaction temperature has obvious effect on carbonization degree, quantum yield (QY), ultraviolet-visible (UVvis) absorption and photoluminescence (PL) spectra but less effect on functional groups, nitrogen doping degree and fluorescence lifetime of NCDs. Compared with N-free CDs-180, NCDs-180 possesses enchanced QY and longer fluorescence lifetime. Doping nitrogen has obvious effect on UVvis absorption and PL spectra but less effect on particles sizes and carbonization degree. The formation mechanism of NCDs is explored: QY of NCDs depends largely on the number of fluorescent polymer chains (FPC), the competition between FPC formation on the surface of NCDs and carbon core growth leads to the change in number of FPC, and consequently to the NCDs with highest QY at appropriate hydrothermal temperature. (C) 2016 Elsevier B.V. All rights reserved.