The turbulent flow in a duct with curvature was investigated by measuring the three dimensional flow fields. Because of the secondary flow influenced by centrifugal force in a curved duct, the effect of duct contraction and expansion were examined. The cross sectional size of the rectangular duct was 120mm by 2４0mm, with a curvature of 200°. The experiments were conducted for three duct geometries: a constant duct width （the standard case）, a contraction in duct width, and a duct expansion. The changes in duct width took place from the latter half of the duct curvature as it transitioned to the straight duct section. The results indicated that for the case of an expanding duct, the velocity defect flow was larger, turbulent intensity increased, and the turbulent boundary layer thickened and expanded in three dimensions, as compared with the standard straight duct case. When the flow was contracted, the turbulent intensity and the turbulent boundary layer thickness both decreased, with greater uniformity of the velocity profiles, as compared with the standard case.