This paper is on the cavitation erosion (CE) behavior of a thermally annealed duplex stainless steel having a triplex microstructure consisting of ferrite, austenite and sigma phase. As sigma forms, it siphons off alloying elements from ferrite, making it softer. Ferrite is the weakest against CE among the three phases, and its interfaces with sigma phase are often the initiation sites of damage which tends to grow into the ferrite side. The weakness of ferrite stems from its high strain-rate sensitivity. Austenite is beneficial because it helps to prevent sigma phase from being dislodged, and its interfaces with sigma phase are also more resistant to CE damage. Mechanical properties such as hardness and strength do not correlate with CE resistance (as measured by cumulative weight loss and the incubation time of CE damage). Preliminary results show that cumulative weight loss is not monotonically proportional to the quantity of sigma.