Objective To observe the expression patterns of islet-1(ISL1) in developing mouse embryonic hearts and the morphogenesis of the heart, the second heart field and foregut endoderm. Methods Serial sections of eighteen mouse embryonic hearts from embryonic day(ED) 8 to ED 13 were stained immunofluorescently or immunohistochemically with antibodies against myosin heavy chain(MHC), ISL1, proliferating cell nuclear antigen(PCNA) and α-smooth muscle actin(α-SMA). Expression of ISL1 was measured with Western blotting in mouse embryonic hearts from ED11 to ED14. Results At ED9, ISL1-expressing cardiac progenitors was detected at the distal wall of the outflow tract (OFT). At ED 10, ISL1 positive cells extended into the proximal wall of OFT from the distal and distributed in the myocardium of venous sinus. From ED11 to ED12, ISL1-expression reached the highest level in embryonic hearts. ISL1 positive cells were mainly distributed at the sinus node and the venous valves on venous pole of the heart but were detected at the distal wall of OFT, the developing intrapericardial aorta, pulmonary trunk and aortic-pulmonary septum on the arterial pole of the heart. At the same time, the length of solid corn stretching out from foregut endoderm reached the longest and was surrounded by ISL1-expressing progenitors of anterior component of the second heart field (aSHF). The density of ISL1 positive cells in aSHF was significantly higher than that of posterior component of the SHF. At ED13, ISL1-expressing progenitors were dramatically reduced both in the heart and the SHF, while the solid endoderm cord was gradually eliminated. Conclusion ISL1 positive progenitor cells are mainly detected from ED9 to ED13 in mouse embryonic hearts. The extent of ISL1 positive cells distribution in mouse embryonic hears is accompanied by the morphogenesis of the SHF and foregut endoderm.