Sina has bought a 9-percent stake in Tudou.
The deal is worth an estimated 66.4-million US dollars.
The agreement comes following a lackluster IPO by Tudou in New York earlier this month
Analysts say growing competition in China's online video market is pushing up licensing costs for content and making it harder to retain users, prompting executives to predict consolidation in the industry.
Portal operators, including companies like Sohu and Sina, are now said to be looking to video ads as a new potential growth driver.
Tudou is China's second-biggest online video company by ad revenue, with 14 percent of the market.
It's main rival Youku leads the market with a 23% share.
