Codon DNA sequence(CDS) of four different citrus species,including sweet orange(Citrus sinensis),satsuma mandarin(C.unshiu),grapefruit(C.paradisi) and lemon(C.limon) were used to analyze the relative frequency of synonymous codon(RFSC),and high-frequency codons were selected by high-frequency codon analysis.The results indicated that the codon preference was little different in four citrus species.