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The move is in line with Mengniu's strategy to focus on the quality of its milk supplies and regain trust in the country, which is roiled by a series of tainted milk scandals.
The transaction will raise Inner Mongolia-based Mengniu's stake in China Modern to about 28%.
Mengniu chief executive officer Sun Yiping said: "The increase of Mengniu's stake in Modern Dairy is to secure both quality and quantity of raw milk sources."
Founded in 2005, Modern Dairy is a raw milk supplier to Mengniu and runs 22 large-scale diary farms in China.
Mengniu sells liquid milk products including UHT milk and yogurt under its name in China. It also produces other dairy products, including milk powder.