NASCAR star Tony Stewart will NOT face criminal charges in the death of Kevin Ward Jr. ... and in a shocking revelation, we now know Ward was high on weed at the time of his death.

TMZ Sports broke the story...the Ontario County Grand Jury in New York decided not to charge Stewart, after reviewing evidence from several accident reconstruction experts.

On the night of his death, Ward Jr. had gotten out of his car and was aggressively marching toward Stewart's car -- when he was struck. Now we know drug tests on the body produced proof Ward was under the influence of marijuana.

There was talk that Stewart was angry with Ward Jr. and may have intentionally veered his car into him -- but apparently the grand jury feels it was nothing more than a freak accident.