Sound radiation from side wall of glass fiber ducts is studied by measuring complex sound intensity in sound field outside side wall. The sound from side wall radiates much more within, about 1 meter in the length of glass fiber ducts from joint with steel ducts. For evaluating sound characteristics of straight glass fiber ducts, nine kinds of test units (round, regular polygon and square section ducts, each of them have three kinds of ducts. ) are studied. By the difference in their duct section types sound attenuation and sound insulation characteristics of side wall are compared and discussed.