The Yinmawanshan gabbro in southern Liaodong Peninsula is one of the youngest plutons in Northeast China. In this paper, the gabbro was investigated by petrologic and geochemical methods and calculated by means of PELE program (a silicate liquid crystallization program). The gabbro has high Al2O3（16.28%～19.27%） and low MgO（3.63%～5.57%）, belonging to high aluminum basalt. The gabbro has the assemblage of olivine + clinopyroxene + magnetite + ilmenite + biotite + plagioclase + K-feldspar. The ophitic and subophitic textures, elongated apatite crystals, low AlⅥ/AlⅣ ratios (0.00 ～0.28, 0.12 on average) in clinopyroxene, and low CaO content in olivine (0.04%～0.24%, average 0.12%) indicate that the gabbro was crystallized at a low pressure. The sequence of mineral crystallization in the gabbro is as follows: plagioclase, olivine, magnetite, clinopyroxene, ilmenite, apatite, biotite, K-feldspar, as revealed by petrography combined with the calculation using PELE program. The gabbro is indicative of intense magmatism in Cenozoic in Northeast China.