Based　on　test　results,　a　new　collapse　mechanism　of　punching　shear　failure　of　concrete　slabs　is　established　in　this　paper.　The　twin　shear　strength　theory　is　used　in　the　plastic　limit　analysis　to　obtain　the　punching　shear　capacity　from　the　failure　cone.　It　is　suggested　that　the　ultimate　punching　shear　strength　of　reinforced　concrete　slabs　consist　of　the　punching　resistance　capacity　of　the　failure　cone　and　a　part　of　flexure　strength　of　the　whole　slab.　The　formulas　are　provided　to　calculate　the　strengths　of　ordinary-strength　concrete　slabs,　high-strength　concrete　slabs,　steel-fiber　reinforced　ordinary-strength　concrete　slabs　and　steel-fiber　reinforced　high-strength　concrete　slabs　subjected　to　punching　shear.　Good　agreement　between　the　present　theoretical　values　and　available　experimental　results　is　achieved.