Recently, the multiple-connected piezoelectric transformer (PT) is considered as significant research topic especially for power processing applications. However, there is a power balance issues closely related to thermal failure of PT. To avoid the thermal imbalance between PTs, a design specification of thermal characteristic parameters of piezoelectric transformer in input-parallel and output-parallel connection has been presented. In first, the individual thermal characteristics of radial-vibration mode PTs are investigated and the results are also the required design area of thermal parameters in parallel-parallel connection by three steps. As an objective function, relative power-loss rate was defined and used for deriving the thermal balance design criteria. The design specification of major design parameters such as thermal parameter variation (Δ Leq,Δ Rm), initial deviation of the variation (δ LeqT,δ RmT), and initial deviation of thermal parameters (δ Leq0,δ Rm0) have been proposed as (-0.002,-0.006), (± 0.0001,± 0.0003), and (± 0.0002,± 0.0004) respectively, and the analysis has been verified by the simulation results.