The Washington Post will purchase the newspaper plant at 225 Virginia Ave. SE that was the home of the defunct Washington Star and begin printing some of its daily press run there, The Post announced last night.

The Post also is buying most of The Star's equipment, including six presses and a fleet of delivery trucks. The former Star plant will become, in effect, a second satellite of The Post's main printing plant at 1150 15th St. NW. Another satellite plant was opened in Springfield last year.

The Star's owner, Time Inc., which folded the 128-year-old evening daily last month, and Washington Post publisher Donald Graham declined to disclose how much The Post agreed to pay for the plant, but informed sources said the price was between $13 million and $15 million.

The property is assessed for tax purposes at $11.75 million, according to D.C. government records. Time Inc. executives were quoted last month as saying that the $7.5 million mortgage that was on the property when they bought it from Joe L. Allbritton in 1978 has been reduced to $6.6 million, but The Post did not assume the mortgage.

Graham said daily and Sunday circulation of The Post has grown by more than 100,000 papers in the month since Time Inc. announced it was giving up on The Star. That would put The Post's daily circulation at about 725,000. With circulation and advertising heading for the peak season of the year, he said, the additional printing and distribution capability acquired with The Star plant will enable The Post to "print on time and give good delivery service to our subscribers." The Star's daily circulation was 323,000.

Graham said it is likely that additional production workers would be hired, perhaps from the rolls of those laid off when The Star folded, and that some would resume working full-time at the former Star plant.

He said the decision to buy the plant had "nothing to do" with speculation that The Post would begin publication of a separate afternoon newspaper. A decision on any such venture, he said, is far in the future. The immediate need, he said, was to ensure that The Post could meet the expanding demand for ever largerpapers.

There apparently were no other publishing companies or newspapers interested in acquiring The Star's plant adjacent to the Southeast Freeway. Some potential buyers reportedly approached Time about buying the property for conversion to housing or other commercial uses, but the plant is in an area plagued by crime and cut off from prosperous Capitol Hill by the elevated freeway, and probably would have been worth less as a speculative investment.

Graham said The Star's printing and production equipment are compatible with those at The Post, and that The Star plant is capable of putting out a 128-page daily edition. The presses in The Post's main plant can produce an edition of only 112 pages at a time.

Major assets of The Star not included in the sale, Post officials said, included the name of the paper and its Logicon computerized typesetting system. The Post recently installed its own system, custom designed to Post specifications by the Raytheon Co.

The Star, which began life as a small penny sheet in a print shop at Eighth and S streets NW, had several homes during its lifetime. The Virginia Avenue plant was opened in 1959.