Chalcone isomerase (CHI) plays critical roles in plant secondary metabolism, which is important for the interaction between plants and the environment. CHI genes are widely studied in various higher plants. However, little information about CHI genes is available in peanut. Based on conservation of CHI gene family, we cloned the peanut type II CHI gene (AhCHI II) cDNA and genome sequence. The amino acid sequence of peanut CHI II was highly homologous to type II CHI from other plant species. qRT-PCR results showed that peanut CHI II is mainly expressed in roots; however, peanut CHI I is mainly expressed in tissues with high content of anthocyanin. Gene duplication and gene cluster analysis indicated that CHI II was derived from CHI I 65 million years ago approximately. Our gene structure analysis results are not in agreement with the previous hypothesis that CHI II was derived from CHI I by the insertion of an intron into the first exon. Moreover, no positive selection pressure was found in CHIs, while, 32.1 % of sites were under neutral selection, which may lead to mutation accumulation and fixation during great changes of environment.