SpaceX's launch director gave the green light on Saturday to fuel the rocket for an historic flight to the International Space Station by two veteran NASA astronauts.
"We're go for propellant load," launch director Mike Taylor said.
The fueling of the Falcon 9 rocket with rocket grade kerosene and liquid oxygen began 45 minutes before the scheduled 3:22 pm (1922 GMT) blastoff of the Falcon 9 rocket and its Crew Dragon capsule with astronauts Robert Behnken and Douglas Turley on board.
Wednesday's launch was postponed because of poor weather just 17 minutes before liftoff and weather conditions have been uncertain on Saturday as well.
SpaceX is seeking to become the first private company to send astronauts into orbit in what would be the first crewed space flight from US soil since the space shuttle program ended in 2011.