In this study,a Populus tomentosa cytosolic APX gene,named as PcAPX,was identified and characterized.The recombinant PcAPX protein,encoding a 249-residue protein with a calculated mass of 33.01 kD,showed high activity towards ascorbic acid (AsA) and H₂O₂.At fixed H₂O₂ concentration,the K_m and V_max values were (0.71±0.03) mmol·L^-1 and (0.41±0.02) mmol·L^-1·min^-1·mg^-1 for AsA.And at fixed AsA concentration,the K_m and V_max values were (0.60±0.21) mmol·L^-1 and (0.35±0.12) mmol·L^-1·min^-1·mg^-1 for H₂O₂.These data showed that the protein was active with AsA and H₂O₂ and had high catalytic efficiency.Real-time PCR analysis showed that the expression levels of PcAPX was higher in the mesophyll of aged leaves than that of other tissues.This research provided the theoretical support to investigate roles of APX gene family in plant growth regulation.