The　failure　characteristics　of　layered　rocks　are　a　vital　issue　for　stability　evaluations　of　underground　engineering　projects.　Layered　rocks　in　a　restricted　true　three-dimensional　(3D)　stress　state　behave　substantially　differently　from　those　under　uniaxial　or　conventional　triaxial　stress.　In　this　study,　true　triaxial　experiments　are　carried　out　to　investigate　the　mechanical　and　volumetric　fracturing　behaviors　of　layered　composite　sandstones　(LCSs)　with　a　change　in　grain　size　(Cgrain　size)　between　the　layers.　Under　true　triaxial　stress,　the　strengths　of　LCSs　increase　considerably　as　the　Cgrain　size　value　increases,　which　is　first　reported.　The　strength　enhancements　of　LCSs　with　increased　σ2　(intermediate　principal　stress)　and　Cgrain　size　values　are　related　to　the　enhanced　grain　interlocking　effect　and　the　interface　effect　between　the　layers.　As　σ2　increases,　the　volumetric　fracturing　behaviors　of　LCSs　change　from　3D　failure　toquasi-3D　failure　characterized　by　delamination　fracture.　The　effects　of　σ2　on　fracture　are　revealed　by　quantifying　the　cracking　morphologies　on　the　layer　interface.　Aided　by　the　nuclear　magnetic　resonance　(NMR)　technique,　the　correlations　of　rock　porosity　with　the　σ2　and　Cgrain　size　values　are　established.　This　paper　promotes　the　understanding　of　the　failure　mechanisms　of　LCSs　with　a　contrast　in　grain　size　under　true　triaxial　stress　and　helps　interpret　field　observations.　©　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Austria,　part　of　Springer　Nature　2024.