Ordinary bus 3 or air-conditioned bus 524 go to the Northern Bus Terminal (Mo Chit) and passes by Chakrabongse Road (at the western side of Khao San Road). This bus stops near Chatuchak Weekend Market. The trip costs 6.50 baht and takes approximately 30 minutes. http://wikitravel.org/en/Bangkok/Khao_San_Road
http://wikitravel.org/en/Bangkok/Khao_San_Road