McChrystal cited as evidence a recent trip to Afghanistan's southern Helmand province where many of the US reinforcements will be sent.
"When I sit in an area that the Taliban controlled only seven months ago and now you meet with ... elders and they describe with considerable optimism the future, you sense the tide is turning."
But he also said the battle is not yet over.
"It's not a completed mission yet. I think Americans and all our coalition partners need to understand that this will be difficult.
"I won't predict causality numbers but I do believe that there will be a difficult struggle with the insurgency in the months ahead."
McChrystal's request for an additional 30,000 was approved just last month.
Obama announced in December that he would send the reinforcements to Afghanistan, but also said they would begin to pull out by mid-2011.
McChrystal's comments came on the deadliest day for Nato troops in Afghanistan in two months.
Six international soldiers were killed on Monday in various incidents around the country.
Nato officials said the dead included three Americans killed in southern Afghanistan, one French soldier who died in the northeast of the capital, Kabul, and two others whose nationalities were not officially given.
Nato and the US have 113,000 troops in Afghanistan fighting al-Qaeda and Taliban-led fighters, who are aiming to overthrow the government.
Foreign troops have been in Afghanistan since 2001 when the US and UK invaded the country in order to remove the Taliban, who were accused of harbouring al-Qaeda operatives, from power.
But Taliban and al-Qaeda have both regrouped since then and continue to launch attacks.