The placement and thickness of the Faraday shield in a high-frequency coaxial transformer (HFCT) has to be optimized as it has a significant influence on the intra- and inter- capacitances. This paper presents the parasitic capacitance matrix of a HFCT with and without a shield calculated by the Finite Element method. A capacitance formula is developed for the HF equivalent circuit. The simulation results demonstrate that the insertion loss caused by the shield can be neglected, and the intra-capacitance is almost reduced to zero if the shield is grounded properly. The calculated inter-capacitance of the HFCT is in good agreement with the experimental result. An optimized winding arrangement is used to minimize the power loss in the HFCT.