Four new mixed ligand transition metal (Co(II), Ni(II), Cu(II) and Zn(II)) complexes of p-coumaric acid and coumarine have been synthesized and the structurally characterized. These complexes contain hydrate water except the Cu(II) complex. Coumaric acid is a monodendate ligand except Cu(II)-complex in which a bidendate coordination mode could be observed. The thermal decomposition starts with dehydration of Co(II), Ni(II) and Zn(II) complexes while in Cu(II) complex decomposition starts with fragmentation of organic ligands at 145 °C. Studies on their biological activity have also been performed. Molecular formulas of the obtained complex are as follows: (I) [Co(C9H7O3)2(C9H6O2)2(H2O)2].4H2O; (II) [Ni(C9H7O3)2(C9H6O2)2(H2O)2].6H2O; (III) [Cu(C9H7O3)2(C9H6O2)2] and (IV) [Zn(C9H7O3)2(C9H6O2)2(H2O)2].3H2O. The ratio of metal: p-coumaric acid: coumarin in complexes are found to be 1:2:2. The complex IV is diamagnetic, and the other complexes are paramagnetic. According to BM data and result of electronic spectra, the geometry for all of the complexes has thought to be octahedral coordination around the metal ions.