A structure of a holding mechanism with a rotatable bidirectional knob suitable for securing a radiator is disclosed. The holding mechanism comprises a pressure-bearing unit, a buckling element and an operation assembly. The buckling element and the pressure-bearing unit have a positioning opening respectively. The pressure-bearing unit provides a slot for being penetrated with the buckling element. The operation assembly is pivotally positioned on the buckling element and comprises a bidirectional knob and a protrusion member. The protrusion member presses against the pressure-bearing unit while the bidirectional knob being turned. The buckling element is movably disposed at and of the pressure-bearing unit to effectively secure the radiator in place.