The Chinese traditional maritime boundary line (U-line) in the South China Sea (SCS) and its validity under international law have been challenged by bordering states and some scholars. In 2012, Franckx and Benatar questioned the validity of the U-line in view of factors such as the intent of the state reflected by the map, the cartographer's neutrality, technical accuracy, consistency of the cartographic material, and recognition and acquiescence by the international community. This paper points out that the SCS U-line drawn by the Chinese government in 1947, at the median position between China's SCS islands and reefs, and the coastlines of bordering states, reflects the scope of China's claims and intent. The consistency of the claims has been maintained by the PRC after 1949, and the claims have been recognised or acquiesced to by bordering states over quite a long period. Therefore, the U-line has probative force under international law.