In a frankly acknowleged effort to lessen Romania's economic dependence on the Soviet Union and increase its political links to the West, the Bucharest government recently announced its surprise decision to purchase a third interest in a Buchanan County, Va. coal mine.

It is believed to be the first important investment in the United States by a Communist government.

Beginning with a $53 million advance payment for coal deliveries, Romania's share of total investment costs in the Island Creek Coal Co. in southwestern Virginia could eventually rise to as much as $1 billion.

Romania imports 40 per cent of its consumption of coking coal, a vital resource for the steel industry here. Most of the imports now come from the Soviet Union.

"I did have to do a lot of explaining here why a developing Socialist country, which has a per capita income of $1.000, should invest in the United States," Ion Patan, foreign trade minister and deputy prime minister, said in an interview.

"I explained that it was a good investment for a resource we need, and that it was part of a process of increasing the understanding between the peoples of our countries," Patan said.

In a separate interview, President Nicolae Ceausescu called the Island Creek investment "important" and said that "if the contitions are adquate, we are prepared to engage in such investments in the United States, or to permit U.S. ventures here in Romania."

"important" and said that "if the conditions are adetal Petrleum Corp., owned by Armand Hammer, who has been a pioneer in large-scale trade with the Soviet Union.

A second part of the agreement, which was signed early last month, propose an exchange of experts and technologies between Occidental and Romania for off-shore oil exploration in the Black Sea and secondary recovery techniques for Romania's declining oil production.

Patan firmly rejected suggestions that Occidental personnel would undertake any exploration in the Black Sea, however,. The Soviet Union is known to regard this area as too sensitive for outside involvement.

Among the details to be negotiated in the second part of the agreement is a plan for joint ventures by the Romanians and Occidental in third countries, especially developing nations. Patan listed Colombia, were Occidental and the Romanians are interested in coking coal deposits, as the first likely country for such an approach.

Current Romanian-U.S. trade exceeds $300 million a year. Patan emphasized that 51 per cent of Romania's trade today is with non-Communist countries.

"We do intend to diversify," he added. "You can't go only on one side in the world today. We have economic relations with every country except South Africa. Rhodesia and South Korea. We have even had problems of boycott because of our trade with Israel."

The deal advances Romania's pioneering efforts in jointly owned enterprises with American companies. In 1973, Romania and the Control Data Corp. of Mineneapolis established a company here for assembling computer equipment. Romanian officials complain that American government reluctance to export electronic components to this Communist nation has prevented the $4 million investment from becoming profitable.

Occidental asserts that the Island Creek venture will provide about 500 new jobs in southwestern Virginia. Romania has contracted to buy 35 per cent of the mine's production, getting about one-half of its annual purchases of one million tons at cost.