Hi @zhengsy,
The IFC format has a complex structure for both geometry and attributes, and needs quite a bit of manipulation in order to convert to 3D formats other than 3D PDF.
FME's Revit reader will read IFC files as well, and has Data Views that simplify the IFC data for easier conversion to GIS or 3D formats.
I would recommend using the Revit reader with the 'Building Elements with Hierarchy' to read your IFC file. This will provide you with 3D features with the Property Sets as attributes.
The IFC features can be written out to separate OBJ files by creating more than output Feature Type, or using Feature Type Fanout.
You can write the attributes out to Excel or CSV to preserve them. In this case you will need to write each feature out to a separate OBJ file, and include that file name in the Excel data in order to link the attributes to the record.
When writing to OBJ, FME uses the dissolve token ('d') instead of transparency ('Tr'). For the dissolve token, 1 is opaque and 0 is fully transparent. You can find more detailed material specifications at http://paulbourke.net/dataformats/mtl/.