Beijing has said that Taiwan's domestic politics do not change the "fact" that it is part of China, and warned that the island's independence efforts are a "dead end", AFP reported.
The comments come just hours after Taiwan's new president, Lai Ching-te, was sworn in.
China considers Taiwan part of its territory and has long threatened to use force to bring the island under its control.
He described Lai as a "dangerous separatist" because of his past comments on Taiwan independence - rhetoric he has toned down in recent years.
Asked about his inauguration, Foreign Ministry spokesman Wang Wenbin said Taiwan's independence was a "dead end."
"No matter under what guise or flag, the quest for independence and secession of Taiwan is doomed to failure. No matter how the domestic political situation in Taiwan changes, it will not change the historical and legal fact that the two sides of the strait belong to one China. The day of China's full reunification will eventually come," he added.
As Lai took office, Chinese state media reported that Beijing had imposed sanctions on three US defense companies over their arms sales to Taipei.
Chinese social media network Weibo also blocked hashtags related to the inauguration.
Before the inauguration, the Taiwan Affairs Office in Beijing said that "Taiwan independence and cross-strait peace are like water and fire."
In his speech, Lai directly addressed the threat of war after years of mounting pressure from China to bring Taiwan under mainland rule.
Lai said the "glorious era of Taiwan's democracy" had arrived and thanked citizens for "refusing to be influenced by outside forces, for resolutely defending democracy." /BGNES