Gonadotropin releasing hormone (GnRH) is a neuropeptide known to regulate reproduction in vertebrates. Different analogues of synthetic GnRH are used to induce final sexual maturation in fish breeders. The purpose of this research was to evaluate the expression of recombinant GnRH (rGnRH) in Escherichia coli BL21 to produce recombinant hormone. In the present research, the sequence of DNA related to designed fish GnRH was cloned in pET28a⁺. The cloning was confirmed by polymerase chain reaction (PCR) and specific primers. Recombinant vector pET28a⁺/GnRH was transformed into the expression host, E.coli BL21 (DE3). The transformation was confirmed using colony PCR. The transformed bacteria were cultured in LB medium containing kanamycin antibiotic at 37°C at overnight then, induced with 1 mM IPTG. After induction, the bacteria were cultured in two different times and temperatures including 37°C for 6 h and 20° C for 24 h. The protein expression was determined by SDS-PAGE analysis. A band of expected size (186 base pair) of amplification from the gene transmitted to the bacteria was detected on the agarose gel. The 8-kD band of peptide expression was observed in the SDS-PAGE gel and the expression level in 20° C for 24 h was higher than 37° C for 6 h.  The result showed that GnRH from fish had the possibility of producing in the prokaryotic expression system and after optimization, it can be introduced as a specific homologous for the treatment of reproductive disorders in fish.