We have grown vertically aligned amorphous silicon nanowires on Au-Pd co-deposition silicon oxide substrate by thermal chemical vapor deposition using SiH4 gas at 800 degreesC. The diameter of silicon nanowires is in the range 10-50 nm and the length is about 1 mum. Transmission electron microscopy (TEM) observations show that the grown silicon nanowires are of an amorphous state and some of nanowires appear to bifurcate in the vertically growth process. The effect of Hz gas etchings on the catalytic size and the effect of catalytic size on the formation of the vertical growth nanowires are discussed.