Abstract: Shear strength of saturated MSWI bottom ash increases with age due to its activity. In this paper, shear strength of saturated MSWI bottom ash is measured using triaxial tests. The confining pressures are 50kPa, 100kPa, 200kPa and 400kPa, respectively. The dry densities of tested samples of saturated MSWI bottom ash are 1.4g·cm^-3, 1.5g·cm^-3and 1.6g·cm^-3.Their ages are 3d, 7d, 14d and 28d, respectively. The results of triaxial tests show that the failure modes of saturated MSWI bottom ash in triaxial tests are affected by cured ages and confining pressures. The samples with age less than 14d undergo brittle failure at confining pressure less 200kPa. The envelope of Mohr-Coulomb criterion is not a straight line. Two straight lines exist in the plane of σ₁-σ₃ at demarcation point of σ₃=200kPa. Thus, the regressing maximum principal stress with minor principal stress is suitable to estimate shear strength parameters using triaxial tests.