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Health conditions of steel strands influence directly the durability and security of whole prestressed structures. Advanced ultrasonic guided wave technique is used for defect detection of multi-wire steel stands with help of new type magnetostrictive transducers. A kind of special magnetostrictive transducer is developed for the excitation and reception of the lowest longitudinal mode L(0,1) at 160 kHz which is suitable for defect detection of steel strands. Artificial notches in helical and central wires of steel strands which nominal diameter is 17.80 mm are detected using L(0,1) mode excited and received by a pair of magnetostrictive transducers. For analyzing the sensitivity to the notches with variable depth in different wires, the experimental relation curves between the amplitudes of notch echoes and the notch depths are obtained. Experimental results show that ultrasonic guided waves can be used for the inspection of whole steel strands. Furthermore, from the amplitudes of defect echoes in steel strands, it is obviously found that L(0,1) modes is much more sensitive to the defects in helical wires than ones in central wires although the sizes of these defects in different wires are almost same.