I know some wireless card can work in Master Mode but it seems use madwifi driver.

I know madwifi doesn't support usb dongles

I hope to find a usb wireless card which can work as AP under Ubuntu.

any one have suggestion?

Thanks a lot for help.

I have check this list
http://atheros.rapla.net/

I am not sure if ar9170usb.ko and its device support Master MOde.