High quality single crystal Si nanowires with diameters ranging from 10 to 200nm have been successfully grown by using Al catalyst via vapor-liquid-solid mechanism. Critical issues such as the effects of surface oxidation of Al seeding layer on the growth of Si nanowires and selective removal of metal- catalyst after nanowire growth have been systematically studied. Results show that the growth of Si nanowire is strongly dependent on the surface oxidation of Al seeding layers. It is also found that metal-contamination-free Si nanowires can be achieved by selective etching of remaining Al particles from nanowires, providing a Si nanowire as a CMOS compatible building block for future nanoelectronics applications.