In recent years, demands for high-speed/high-bandwidth communication for in-vehicle networks are increasing. This is because the usage of high-resolution displays and high-performance Rear Seat Entertainment (RSE) System are expanding, and also due to the increase of Advanced Driver Assistance Systems (ADAS) and introduction of autonomous driving system in the future. Large-volume data such as high definition sensor image or obstacle information are necessary in order to realize these systems. As a consequence, Automotive Ethernet which meets the requirements of high-speed/high-bandwidth communication attracts many attentions. When Automotive Ethernet applies to in-vehicle networks, it is necessary for technology developments to satisfy our EMC performance requirements. In-vehicle EMC requirements comprise of two parts, 'Emission' and 'Immunity'. 'Emission' requirement is to restrict the electro-magnetic noise emitted from vehicle. Therefore Automotive PHY transceiver that suppresses emission noise has been developed and are available on the market. 'Immunity' requirement is the resistance against electromagnetic radiation from in/outside the vehicle. It is commonly known that the immunity performance decrease when the impedance character of transmission line is unbalanced. In the vehicle circumstances, for example circuit pattern length of the ECU and characteristics of the electronics parts used in ECU affect the impedance balance of the transmission line. In this paper, we propose the technique of ECU circuit design management by using mixed mode S parameter characteristics. And we validated that the mixed mode S parameter characteristics correlate with resistance performance against the immunity noise. So we have defined the criteria of 100BASE-T1 PHY's mixed mode S parameter to satisfy this performance. And we have tried to define the 1000BASE-T1 PHY's criteria and S parameter test method.