This article discusses characterization and operational envelope of an auxiliary electric transmission oil pump (eTOP) in a vehicle transmission system (VTS). The eTOP supplied by a DC voltage consists of a mechanical pump assembly with inner and outer rotor, a permanent magnet (PM) motor and a voltage source inverter (VSI). The PM motor supplies required torque and speed for the mechanical pump assembly whose output is an oil flow at a specified pressure. The eTOP in the VTS is used for different functionalities such as lubrication and cooling of gears and/or providing pressure for clutches and actuators. The eTOP operational envelopes are extracted for different loads, i.e., pressure and flow, and for different VTS environment temperatures. Characteristics are investigated when the eTOP operates continuously at normal condition and when the vehicle operates on New European Driving Cycle (NEDC). The simulated performance and characteristics are validated via measurements from a prototype system.