Parasites for living in host’s body, evade from host’s immune system using variant methods such as production of some molecules with immunosuppressive properties. Of these immunosuppression molecules are morphine and morphine like substances. It can effect on innate and adaptive immunity and also humoral and cellular immunity. The past researches were approved the existence of morphine in the brain and adrenal of beef and in brain of mammalian and also it has been demonstrated as an immune modulator molecule in some invertebrates, some of the nematodes and also a trematode Schistosoma mansoni, But there is no document for morphine evidence in trematode Dicrocoelium dendriticum. Infection with this trematode has much importance either in health of ruminant and economic loos in the meat industry. In addition, this trematode is a zoonosis that there is no proper treatment for it in contrast with the other liver fluke, Fasciola hepatica. We assume that living for a long time in the bile ducts of host is related to morphine like substance as an immune regulatory molecule. We use RP-HPLC method for detection morphine in D. dendriticum and also using reverse transcription polymerase chain reaction (RT-PCR) for μ opioid receptor finding. According to the results D. dendriticum has endogenous morphine like material and also the existence of opioid μ receptor in this trematode was approved. Living in host bile duct for a long time without sever immunopathologycal impact, suggested that probably this worm use of morphine like material as an immunoregulatory molecule and also for escaping from host immune system.