Video footage from Thursday's concert in Edmonton shows the singer, whose real name is Marvin Lee Aday, falling on stage during his performance, reported BBC News.
Earlier in the week he had cancelled shows in Moose Jaw and Calgary, saying he was ill.
The 68-year-old is one of the most successful recording artists of all time with hits including "Bat Out of Hell".
He was performing another hit, "I'd Do Anything for Love (But I Won't Do That)", when he dropped his microphone and fell to the floor, video footage showed.
More From This Section
Fans said they initially believed his collapse could have been a planned part of his performance, the Edmonton Journal newspaper reported.
The Northern Jubilee Auditorium in Edmonton was cleared shortly afterwards and ambulances were seen outside the venue, the newspaper said.
Loaf previously collapsed in 2003 at a performance in London, and again in 2011, during a July concert in Pittsburgh, an incident which he later blamed on an asthma attack.