Heparin-immobilized Pluronic (F-68)/Polyvinylalcohol (PVA) composite microparticles were designed and characterized for the sustained drug delivery of ionic drug. Venlafaxine, antidepressant medication, was used as a model drug. For the efficient loading of ionic drug, heparin was immobilized into F-68/PVA composite microparticles. Differential scanning calorimetry (DSC) was used to understand the intra/intermolecular interactions in the heparin-immobilized F-68/PVA composite gels containing model drug. For the application as a sustained drug delivery system, the loading amount and release pattern of loaded drug were measured using high performance liquid chromatography (HPLC).