Since the lined seahorse Hippocampus erectus was introduced into China in 2009, it has become the major aquacultured seahorse species in China. With the development of its aquaculture, the genetic quality of the species should be evaluated to provide fundament for artificial breeding study. The karyotype is varied among species, and is directly related to the genetic quality. Thus karyotype is the basis for identifying taxonomic status and study on phylogeny of a certain species. Study on karyotype would contribute to the studies on the genetic identification and community analysis. In order to explore the cytogenetical characters of the lined seahorse H. erectus, the metaphase chromosome was obtained from the dorsal fin of six-month old fish by colchicine incubation and hot air drying methods. Clear images of the chromosomes indicate the dorsal fin is an appropriate tissue for chromosomal investigation in seahorses. There were 22 pairs of chromosomes in diploid (i.e. n=22, and 2n=44). The karyotypes of both sexes were examined separately. There were significant differences in karyotype between males and females, male karyotype formula is 2n=2sm+20st+22t, and female karyotype formula is 2n=1m+2sm+20st+21t. Heterotypic sex chromosome was found in female H. erectus, so its sex chromosome belongs to ZW/ZZ type. The preparation of chromosome, karyotype and heterotypic sex chromosome in the lined seahorse were reported for the first time. The present study lays a good foundation for studies on cytogenetics, genetic breeding and protection of genetic resources of the lined seahorse.