Three　models　of　3D　cast-in-place　RC　frame　structures　are　simulated　in　ABAQUS.　In　the　three　models,　one　represents　the　actual　RC　frame　structure　with　monolithic　slab,　one　represents　the　open　frame　structure　used　in　structural　design　currently,　which　means　truss　structure　at　the　stage　of　calculating　internal　forces　of　frame　beams　and　columns,　and　the　third　one　represents　the　open　frame　structure　which　takes　into　account　some　slab　reinforcement　in　the　rectangle　section　of　frame　beam　to　consider　the　slab　participation　in　the　flexural　capacity　of　frame　beam.　By　investigating　the　failure　mode　of　these　three　models,　the　development　of　plastic　hinges,　the　reinforcement　stress　of　beam,　column　and　slab　in　the　models,　the　influence　of　monolithic　slab　to　the　bending　resistance　of　beam　end　is　studied.　It　shows　that　the　existence　of　slab　makes　the　failure　mode　of　actual　RC　frame　structure　different　from　the　design　criteria　of　strong　column　weak　beam.　It　also　indicates　that　the　slab　participation　in　the　bending　resistance　of　beam　end　is　not　only　expressed　in　the　slab　reinforcement　but　also　in　the　structure　integral　performance.　Besides,　when　using　the　effective　flange　width　to　consider　the　slab　reinforcement　participation,　the　structure　integral　performance,　lateral　displacement　and　other　relevant　factors　should　be　taken　into　account.