We have measured the elastic constants of duplex stainless steel, JIS-SCS14A (CF8M), aged isothermally at 400 o C up to 10,000 h, using resonant ultrasound spectroscopy method. The elastic constant c 1 1 (=λ+2μ) increases monotonically with the aging time, but c 4 4 (=μ) remains virtually unchanged. Using a micromechanics model, we have deduced the elastic constants of α-phase on the assumption that those of γ-phase are unchanged with the aging. The increase of the α-phase elastic constants reflects the spinodal decomposition of α-phase.