Monotonic and cyclic triaxial tests were performed on decomposed granite soil (Masado) sampled from Kagawa Prefecture. The effects of the degree of compaction (Dc) and hydrogen peroxide (H₂O₂) injection on the shearing properties of this soil were considered. The chemical weathering of Masado was simulated by reacting the soil with H₂O₂ to oxidize it. The present study was conducted to understand the effects of soil compressibility and chemical weathering on the monotonic and cyclic undrained shear behaviors of soils. In addition, a series of undrained cyclic triaxial tests, followed by monotonic loading, were conducted on the soil to evaluate the post-liquefaction behavior. The cyclic shear strength of Masado at Dc = 100 % tended to decrease with increasing H₂O₂ injection. For the postliquefaction characteristics of Masado at Dc = 100 %, as liquefaction resistance factor FL decreased, the stiffness in the small-stiffness region declined and strain in the low-stiffness region increased. The behavior after liquefaction was only slightly affected by chemical weathering.