This study evaluated the solid particle erosion characteristics of 2.25Cr-1Mo steel with aging time. Aging was performed at 750 °C until 100 h. Specimens aged at each time were characterized by microstructure analysis and Micro-Vickers hardness. An erosion experiment was conducted using 100~200 μm of stainless steel shot at a flow velocity of 20 m/s for 4 h. A consequently, a microstructure degradation phenomenon in which Cr-rich carbide was coarsened occurred, and the surface hardness decreased by 45%. With a decrease in the hardness, the solid particle erosion damage increased and the erosion damage type changed.