An ultrasound transducer has a cup-shaped casing. The cup-shaped casing has a sidewall. A cross section of an inner surface of the sidewall being perpendicular to a central axis of the ultrasound transducer substantially shows a dumbbell-shaped contour. The dumbbell-shaped contour has a pair of opposite straight-line sections being substantially parallel to a longitudinal axis of the dumbbell-shaped contour and a pair of opposite arc sections being located respectively at two ends of the longitudinal axis of the dumbbell-shaped contour and respectively forming two enlarged portions of the dumbbell-shaped contour. Two ends of each arc section of the dumbbell-shaped contour respectively joint the straight-line sections through two curve sections and each curve section is connected to one end of the arc section and one straight-line section. At least one enlarged area of a dumbbell-shaped surface of an inside bottom surface has a recessed portion.