The current launch has been postponed twice and is now tentatively set for February 27, following an assessment of shuttle flow control valve testing.
"More time was needed to complete analysis and testing," said Allard Beutel, a spokesman with NASA's Kennedy Space Center in Florida.
NASA is concerned over the damage of a flow control valve in shuttle Endeavour during its mission in November. As a precaution, Discovery's three gaseous hydrogen valves have been removed and reinstalled.
NASA said Friday it will hold a news conference on February 20, following a review of space shuttle Discovery's readiness for flight.
The Discovery's 7-men crew includes Japan's Koichi Wakata, who will replace returning ISS engineer Sandra Magnus.
Eight more flights to the station remain before the space shuttles are retired in 2010 and all transport services to the ISS are carried out by the Russian Soyuz and Progress spacecraft at least until 2013.
