Heterotrophic nitrifier with high ammonium and nitrite nitrogen removal ability was isolated from the aquaculture pond，and the nitrification performance of mixed culture of multi-strains was studied． The strains were isolated from sediment，waters and intestinal tract of fish samples in the high density aquaculture ponds by enriching with NH ₄ Cl and NaNO ₂ as a sole nitrogen source respectively and identified by sequence analysis of 16S rDNA． Ammonium and nitrite nitrogen removal ability was investigated under aerobic conditions． The selected strains were quantitatively mixed to evaluate the ammonium removal efficiency under different culture conditions such as carbon source，C/N ratio，initial pH，and NaCl concentration． The nitrifying characteristics were investigated of single， pairwise，three-member microbial community under the optimal conditions． Eight heterotrophic nitrobacteria were isolated and three strains showed the higher ability of removing ammonium and nitrite nitrogen belonged to Bacillus megaterium (W3-1)，B． subtilis (YZN-2) and Lactobacterium plantarum subsp． (HT1-1)，respectively． The NH ₄ ⁺ -N removal efficiency was 71. 2%，61. 3% and 60. 7%，while NO ₂ ^－ -N removal efficiency was 38. 7%， 35. 6% and 37. 6%，respectively． After the optimization of mixed culture，the results show that under the condi- tions of sodium citrate as carbon source，C/N for 20，NaCl concentration for 5 g/L and pH 6，the mean ammoni- um removal rate is 2. 05 mg/ (L·h ^－1 ) in 24 hours． In the community of W3-1 + HT1-1 and W3-1 + YZN-2 + HT1-1，the ammonium removal rate is promoted by HT1-1 at the early stage，e． g． the average ammonium re- moval rate of W3-1 alone is 1. 61 mg/ (L·h ^－1 ) during first 9 h，while the rate of W3-1 + HT1-1 and W3-1 + HT1-1 + YZN-2 raise to 2. 51 mg/ (L·h ^－1 )，and 2. 49 mg/ (L·h ^－1 )，respectively． The above results in- dicate that the strains of W3-1，YZN-2 and HT1-1 show higher heterotrophic nitrification ability than others． The ammonium removal rate of B． subtilis is improved by combination with L． plantarum． The research provides necessary theoretical reference for the fast start of nitrification system in the sewage treatment process and the deve- lopment of nitrifier agents．