Abstract: The using condition of codons of chromosome 7 in chicken which code 21 amino acids was studied. Results showed that codons whose number of synonymous codnns was two or three all tended to use one kind of codons except the codons coding TER amino acid, while codons whose number of synonymous codons was four or six tended to use two or three kinds of codons. So it is concluded that among 59 kinds of synonymous codons in chicken, 27 synonymous codons like UUC, CUC, ACG and GUG, etc. were the optimum codons which were all ended by C or G. Thereby, the use of the optimum codons is closely related to the 3^rd base composition.