Experimental investigations of silicon nanowire mobility characteristics on (100) SOI as shrinking nanowire width to sub-10 nm are reported. Accurate mobility estimations by advanced split CV method for 50~1000 nanowires are performed. For the first time, electron and hole mobility in [100]-directed nanowires are studied and compared with [110] nanowires. It is shown that both electron and hole mobility decreases monotonically and electron mobility of [100]-directed nanowire tends to be comparable to that of [110]-directed nanowire as decreasing nanowire width.