Crews from Truro, Falmouth, Helston and St Just are among 50 firefighters called out to battle a large three storey home in Feock.
Mount George Road closed due to the building fire and smoke blowing across the road from B3289.
Police have closed the B3289 as well, due to the smoke.
A fire and rescue spokesman said: "At 10.31 this morning Cornwall Fire and Rescue Service were called to a fire at a 25m x 15m detached domestic property in Mount George Road, Feock.
"Two pumps from Truro, two from Falmouth and one from Helston are on the scene together with water carriers from Helston and Wadebridge and support vehicles from St Dennis and St Just. Up to fifty firefighters are continuing to tackle the blaze.
"No persons have been reported.
"The cause has not yet been determined and a fire investigation will be carried out in due course."
A house in Feock near Truro has been destroyed by fire. People are being urged to avoid the area. pic.twitter.com/1reXioZccQ
— BBC Radio Cornwall (@BBCCornwall) January 23, 2015
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