Abstract: In order to understand the diatom community structure and reveal the pollution status and water quality in Nan2chong section of Jialing River, 15 diatom samples and water quality samples in five cites or counties ofNanchong city,werecollected according to normalmethods, in dry season, level season and rainy season, respectively. Chemical indices inclu2ding COD, TN and TP were measured. The diatom community structure including diatom species composition, cell densi2ties, Shannon2 Weaver diversity index and Lloyd2Ghelardi evenness index were analysed, and the water quality wasevaluated combined with chemical index. The results showed that: (1) 136 taxa,which belonged to 2 class, 10 family and 26 genera were observed (including variety and form). Pennatae diatom species numberswere richer than that of centriatediatom. The percentage of Pennatae diatoms and centriate diatomswere 94.52% and 5.48% to the total diatoms, respectively. Langzhong section had the highest species numbers with 86 and Nanchong section had the lowest species numberswith the number of 48; The cell densities of diatom was in the range of 4500-39000 ind/L with the mean value of 18213 ind/L. Langzhong section had the lowest cell densities of diatom with the value of 12600 ind/L and Nanchong section hadthe highest cell densities of diatom with the value of 23700 ind/L. The diversity index was in the range of 1.05-3.30,with the mean value of 2.44. Langzhong section had the highest diversity indexwith the value of 2. 85 and Nanchong section had the lowest diversity index with the value of 1.80; the evenness index was in the range of 0.22-0.66 with themean value of 0147. Langzhong section had the highest evenness index with the value of 0.55 and Nanchong section hadthe lowest evenness index with the value of 0.35. (2) The diatom species numbers, cell densities, diversity and evennessindex took on noticeable seasonal and horizontal distribution distinction. To seasonal change, the diatom community structure had similarities in level season and dry season,with more species numbers, higher cell densities, diversity and indicesevenness indices than those in rainy season. To horizontal distribution change, from upstream to downstream, the diatomspecies numbers decreased and diversity index and evenness index declined gradually, however, cell densities of diatom increased gradually. The sampling sites at upstream of the city had more species numbers, lower cell densities and higher diversity indices and evenness indices than those at downstream. (3) Species diversity index and evenness index aswell aschemical itemswere used to evaluate thewater quality in Nanchong section of Jialing River. The results consistently indicated that the water quality in Nanchong section of Jialing River belonged toβ2 mesosaprobic Zone all in all and the waterbody possessed certain self2 purification ability. The water quality of Langzhong section was the best of all,which belongedto Oligosaprobid Zone; those ofNanbu, Xinzheng and Peng’ an sectionswere betweenβ2 mesosaprobic Zone and Oligosaprobid Zone,which of Nanchong section was the worst of all, belonging toα2 mesosaprobic Zone. To improve and protect thewater quality of Jialing River, some prevention and cure measures and suggestions were given in the paper.