This included the study of embryonic development of the heart in different lengths of embryos of Sailfin molly (Poecelia latipinna) fish, which is one of the invasive fish species in Iraq and is spread acrosswater bodies, especially in the marshes that are located in the south of Iraq. The fish samples were collected from the AGhazl market in Baghdad province, and dissected to remove the ovary that containing embryos at different lengths using fine forceps. All the samples were fixed using formalin after making a hole in the gas sac, especially in advanced embryos stage. In a 3 mm embryo, the heart was completely formed and the blood vessels and optic cup were clear, while the lens of the eye was beginning to form. In 4-5 mm embryo, it was found that the formation of the atrium and ventricle was developed clearly, in addition to that the appearance of the bulbus arteriosus were detected. Furthermore, In a 7-6 mm embryo, several changes were revealed including that the atrium, ventricle, blood clots containing hematopoietic cells and blood cells are more clearer than in the previous stage. In a 9 mm embryo in length (birth stage), the heart and its parts were clarified than in the previous stage. In a 10-11 mm embryo  (larval stage), it was noticed the completion of the heart development, clarity of the atrium, ventricle, and bulbus arteriosus, as well as development of the epicardium formation, while the myocardium muscles were very clear.