Boy dies after Perth house fire

A four-year-old boy has died after a Perth house fire that also killed his 69-year-old great-grandfather.

The blaze on Rushbrook Way, Thornlie, was reported about 6.15pm on Wednesday.

Police confirmed the boy's death about 5pm on Thursday and say a report will be prepared for the coroner.

Firefighters found the boy and his great-grandfather unconscious in a bedroom of the home and they were taken to Fiona Stanley Hospital.

But the man was pronounced dead on arrival.

The boy was transferred to Perth Children's Hospital in a critical condition.

Neighbour "Darren" said he saw the blaze engulf his friend's home.

"It was pretty bad coming through the window - it popped a roller shutter off the front," he told the Seven Network on Thursday.

"I went over there, but I didn't like looking."

He said he had lived opposite his neighbours for many years.

"(They're a) close family, lovely people. It's a tragedy, I don't know what to say."

The arson squad is investigating, with the cause of the fire yet to be determined.