Graphene appears to be a good candidate in designing tunable devices operating in THz spectral ranges owing to its tunability of conductivity. It can be tuned dynamically via electrostatic doping [1, 2]. Here we propose a tunable terahertz half wave plate based on the hybridization effect caused by coupling interactions between plasmonic resonances in the two coupled graphene nanodisks (CGN). We show numerically that using a period array of coupled isotropic graphene nanostructures in lieu of uncoupled anisotropic graphene nanostructures [3] is an alternative route to achieve polarization conversion, which may lead to some other novel phenomena and interesting applications.