South Korea's POSCO buys Thainox stake for $287 mln

* To expand its presence in SE Asia's stainless steel market
* Thainox stocks hit 6-1/2-year high

BANGKOK/SEOUL, July 7 (Reuters) - POSCO Pcl , the world's third-biggest steelmaker, said on Thursday it was buying a stake in Thainox Stainless Pcl , Southeast Asia's largest stainless steel producer, in a deal worth 305.9 billion won ($287 million).

In a bid to gain a strong foothold in fast-growing Southeast Asia, South Korea's POSCO, which already owns 15.39 percent of the Thai company, will buy the stake from major shareholder the Mahagitsiri family at 2.20 baht per share.

The news sent Thainox shares up more than 10 percent when they resumed trade in the afternoon after being suspended on Thursday morning pending the announcement.

They ended at 2.08 baht, up 9.5 percent. The offer price was 15.7 percent higher than Wednesday's close of 1.90 baht.

POSCO ended down 0.3 percent at 464,500 won.

"The deal aims to tap into the Southeast Asian market and enhance its global competitiveness in the stainless business," POSCO told the South Korean stock exchange.

POSCO said it planned to buy all the remaining shares it did not own in a tender offer at the same price between September and October.

That would value the company at around $440 million, according to a Reuters calculation.

Thailand, where global home appliance makers and Japanese car makers have production bases, is the biggest cold-rolled stainless steel market in Southeast Asia, POSCO said.

Stainless steel demand in Southeast Asia rises 8 percent a year on average and supply falls short of demand, it said.

Analysts were positive on the takeover plan.

"The deal will help POSCO secure a foothold in the Southeast Asian market and compete better with Japanese and other competitors," said Aum Jin-seok, an analyst at Kyobo Securities.

POSCO could cancel the tender if it was unable to get at least 51 percent of Thainox, the Thai firm told the exchange.

POSCO has long been interested in taking over Thainox but talks had stalled due to political instability in Thailand.

There was a general election in Thailand at the weekend and the outcome -- a landslide win for what was the opposition -- may ensure more stability in the near term, analysts say.

In 2009, POSCO bought a 90 percent stake in a Vietnamese stainless steel company, Asian Stainless Corp.

POSCO's Chinese stainless joint venture, Zhangjiagang POSCO Stainless Steel, recently completed the construction of additional facilities that raised its annual capacity to 1 million tonnes from 800,000 tonnes.

That capacity addition will bring POSCO's total stainless steel capacity in South Korea and overseas to 3 million tonnes, making it the market's second-biggest player along with Taiyuan Iron & Steel and just behind Acerinox SA , POSCO said. ($1 = 30.48 Baht) ($1 = 1065.95 Korean Won)