ABSTRACTThe superconducting transition under magnetic field for organic superconductors к-(BEDT-TTF)2 Cu(NCS)2 and к-(BEDT-TTFh)2 Cu[N(CN)2]Br has been analyzed by employing fluctuation renormal-ization theory. From magnetization measurements the interlayer and intralayer Ginzburg-Landau coherence lengths are determined. The effect of the isotope substitution to BEDT-TTF molecules has been studied. The result for resistive measurements is also described.