Phenothiazine is a synthetic tricyclic compound and it is related to the thiazine heterocyclic with S(C6H4)2NH chemical formula. At first, it was used as an anti-helminthic for livestock and humans but application its derivatives revolutionized psychiatry and other fields of medicine. Electronic structure calculations provide the chemical structure of the system and it is answering the question why the structure of molecules are puckered. The Psuedo Jahn-Teller effect (PJTE) is the only source of instability and distortions of high-symmetry configurations of any polyatomic system in nondegenerate states such as phenothiazine and its derivatives. The goals of this study is: founding the reason of the breaking symmetry phenomena, non-planarity and bending instability in phenothiazine and some its derivatives from C2v symmetry in planar configuration to Cs puckered structure by the PJTE (Fig. 1).figureFig.1: The symmetry breaking phenomena in phenothiazine from unstable high-symmetry planar (C2v symmetry) to stable Cs bent structure. To do this, the Molpro 2010 package were carried out for optimization and frequency calculations of thiazine, phenothiazine and two S-oxygenated derivatives of phenothiazine in their planar configuration. An imaginary frequency was observed along b1 normal coordinate in all these compounds. Following the state-average complete active space self-consistent field (SA-CASSCF) wavefunctions have been employed to calculate the APES along the bending normal coordinates and it shown that (1A1+1B1) b1 PJTE problem is the reason of the breaking symmetry phenomena and bending of the C4NS ring in the all under consideration compounds.