The properties of parylene-C thin films deposited on polymer substrates using a modified chemical vapor condensation technique were investigated. The configuration of the polymerization chamber was altered from horizontal to vertical and designed with a circular symmetry in the z-direction to improve the deposition rate. The growth rate of the thin films was improved 2-5 times in a vertical configuration compared to a horizontal configuration. Potential factors responsible for such an improvement include the effect of coincidence between the flow direction of the gases and the direction of the convection in the reaction tube due to buoyancy. The synthesized films have a high transparency (>90%) in the visible region, a smooth surface morphology (Rs<5.0 nm), and amorphous phase structures with the presence of some crystalline domains.