After the RCEP implementation in 2022, trade differences emerged between Japan and South Korea with China, especially in semiconductors. Japan's exports to China declined, while South Korea's grew. This study uses qualitative and quantitative methods to explain post-RCEP semiconductor trade differences, focusing on value chain complementarity, new tariff rates, and U.S. factors. Three conclusions: 1) Within the global value chain, the different positions of Japan and South Korea in the semiconductor sectors compared to China result in varying levels of market complementarity between Japan and China and South Korea and China; South Korea, in particular, exhibits a significantly higher potential market. 2) In terms of reducing tariff barriers under RCEP, compared to Japan, South Korea experienced a more substantial decline in semiconductor tariff rates in the Chinese market. 3) The non-tariff barrier sanctions imposed by the United States on Chinese semiconductors in 2022 influenced trade choices for Japan and South Korea. Japan, being more loyal to the United States, tends to align with U.S. policies, leading to a reduction in semiconductor trade with China.