A SpaceX Crew Dragon spacecraft will fly the astronauts to the ISS on the companya¿s Falcon 9 rocket.
This "Crew Demo-2" mission will mark the first launch of astronauts on an American rocket from American soil since the last space shuttle mission in 2011.
Launch of the mission, dubbed Launch America, is scheduled for 4.33 p.m. EDT on Wednesday.
Demo-2 will be the first crewed mission for the US space agency's Commercial Crew Programme.
NASA and SpaceX on Friday gave a "go" for the Demo-2 mission and on Saturday the crewmates -- Douglas Hurley and Robert Behnken -- took part in a dress rehearsal of pre-launch events.
"SpaceX and @NASA completed a full rehearsal of launch day activities with @AstroBehnken and @Astro_Doug ahead of Crew Demo-2," SpaceX said in a tweet.
Behnken and Hurley began their day in the Astronaut Crew Quarters inside Kennedy Space Center's Neil A. Armstrong Operations and Checkout Building.
The pair put on their black-and-white SpaceX spacesuits, took the elevator down to the ground level and exited through a pair of double doors, where their transport vehicle -- a Tesla Model X -- waited.
With smiles and waves, they climbed in for the 20-minute ride to Launch Complex 39A.
The Falcon 9 rocket and Crew Dragon spacecraft have been in place on the launch pad since May 21.
Behnken and Hurley entered the Crew Dragon by way of the pad's Crew Access Arm and checked their communications systems before the hatch was closed.
The rehearsal concluded with the go/no-go poll for Falcon 9 propellant loading, which normally occurs 45 minutes before launch.
After completing the full rehearsal, Behnken tweeted, "Next time is for real."
The US Air Force 45th Weather Squadron are predicting a 40 per cent chance of favourable weather conditions for the SpaceX Demo-2 mission, NASA said, adding that the primary weather concerns for launch are flight through precipitation, thick and cumulus clouds.