The acquisition includes the 15.3% stake held through a trustee and a direct 20.4% stake in cash, according to UMC.
UMC noted that it aims to ultimately merge with Hejian in compliance with Taiwan's regulations.
UMC shares slid 1.5% to close at NT$13.15 (US$0.44) on the Taiwan Stock Exchange on November 1, after an 1% rally yesterday.