S-nitrosocaptopril　(CapNO)　possesses　dual　capacities　of　both　Captopril　and　an　NO　donor　with　enhanced　efficacy　and　reduced　side　effects.　CapNO　crystals　are　difficult　to　make　due　to　its　unstable　S-NO　bond.　Here,　we　report　a　novel　stable　S-nitrosocaptopril　monohydrate　(CapNO　center　dot　H2O)　that　is　stabilized　by　intermolecular　five-membered　structure,　where　one　H　of　H2O　forms　a　hydrogen　bond　with　O-　of　the　stable　resonance　zwitterion　Cap-S+　=　N-O-,　and　the　O　in　H2O　forms　the　dipole-dipole　interaction　with　S+　through　two　unpaired　electrons.　With　the　chelation　and　common　ion　effect,　we　synthesized　and　characterized　CapNO　center　dot　H2O　that　is　stable　at　4　degrees　C　for　180　days　and　thereafter　without　significant　degradation.　Compared　to　Captopril,　CapNO　showed　direct　vasorelaxation　and　beneficial　effect　on　PAH　rats,　and　could　be　self-assembled　in　rat　stomach　when　Captopril　and　NaNO2　were　given　separately.　This　novel　CapNO　center　dot　H2O　with　low　entropy　paves　an　avenue　for　its　clinical　trials　and　commercialization.