In recent years, the demand for high-speed/high-bandwidth communication for in-vehicle networks has been increasing. This is because the usage of high-resolution screens and high-performance rear seat entertainment (RSE) systems is expanding. Additionally, it is also due to the higher number of advanced driver assistance systems (ADAS) and the future introduction of autonomous driving systems. High-volume data such as high definition sensor images or obstacle information is necessary to realize these systems. Consequently, automotive Ethernet, which meets the requirements for high-speed/high-bandwidth communication, is attracting a lot of attention.The application of automotive Ethernet to in-vehicle networks requires that technology developments satisfy EMC performance requirements. In-vehicle EMC requirements consist of two parts: emission and immunity. The emission requirement is to restrict the electromagnetic noise emitted from vehicle. Therefore, automotive PHY transceivers that suppress emission noise have been developed and are available on the market. The immunity requirement is resistance against electromagnetic radiation from inside or outside the vehicle. It is commonly known that immunity performance decreases when the impedance characteristics of the transmission lines are unbalanced. In the vehicle, the circuit pattern length of the ECU and the characteristics of the electronics parts used in the ECU affect the impedance balance of the transmission lines.This paper proposes an ECU circuit design management technique using mixed mode S parameter characteristics. The correlation of the mixed mode S parameter characteristics with the resistance performance against immunity noise was validated, and the criteria for the 1000BASE-T1 PHY mixed mode S-parameters to satisfy this performance was defined.