Ulan Obo mafic-ultramafic rocks, located along the Inner Mongolia-Da Hinggan Mountains orogenic belt, consist of harzburgite, gabbros and diabase. Geochemical data indicate that serpentinized peridotites are mantle-derived peridotite. Gabbro and diabase are tholeiitic rocks and similar to N-MORB in LILE components. They together consist of ophiolitic complex. Mineralogical analyses reveal that the olivine in peridotite is forsterite(Fo=91.10~91.81), and chromite has high Cr^# and Mg^#(61.04~64.44, 50.40~56.37) and belongs to pecotites. Clinopyroxene occurs in both mafic and ultramafic rocks, it is endiopside in peridotite and augite in diabase and is different in scale from sahlite to augite in gabbros. Orthopyroxene only occurs in harzburgite and belongs to cilinoenstatite. Zircon U-Pb weighted average age of gabbro is 344.5±1.5 Ma, suggesting that the ophiolite formed in early Carboniferous period. Geochemical and mineralogical researches suggest that Ulan Obo ophiolitic complex formed at the fore-arc setting of the Paleo-Asian Ocean in late Paleozoic period. Petrogenesis of Ulan Obo ophiolitic complex shows favorable conditions for chromite deposit formation,and therefore emphasis must be paid to the chromites in further exploration and prospecting.