Photo-induced insulator to metal transitions (PIMT) in quarter filled layered organic conductors ET [bis(ethylenedithio)tetrathiafulvalene]-based salts α-(ET) 2 I 3 , θ-(ET) 2 RbZn(SCN) 4 , and κ-(d-ET) 2 Cu[N(CN) 2 Br] were investigated using ultrafast spectroscopy in the near, mid-infrared and terahertz (THz) regions. In charge ordered salts α-(ET) 2 I 3 and θ-(ET) 2 RbZn(SCN) 4 , an immediate ( < 30 fs) generation of a microscopic metallic state is driven by the electronic process. Subsequently, condensation of the microscopic metallic domain to the macroscopic scale is accompanied by a small molecular rearrangement in α-(ET) 2 I 3 . However, in θ-(ET) 2 RbZn(SCN) 4 , a large structural difference between the insulator and metallic phases prevents stabilization of the macroscopic metallic state. In a dimer Mott insulator κ-(d-ET) 2 Cu[N(CN) 2 Br], photogeneration of the metallic state shows a finite rise time of ca. 1 ps, which is attributable to the photo-induced change of on-site Coulomb energy on each dimer (U dim ) through dimeric molecular rearrangement. Thus, the ultrafast dynamics of PIMT depend strongly on the molecular arrangement in the layer of ET salts.