Starting in Tavistock, it heads south-westwards towards Liskeard, crossing over the River Tamar and into Cornwall, then through Gunnislake and Callington.

It diverges from the A38 at Dobwalls, where it then runs in a south-westerly direction to Truro via Lostwithiel, St. Blazey, and St Austell.

It then forms an upside down elongated square loop, and bypasses Truro City Centre.

It passes the Royal Cornwall Hospital and skirts the village of Threemilestone, before linking up with the A30 road at Chiverton Cross, where the A390 terminates.

[1] It forms part of the main route (the Truro to Dobwalls section) from Truro to Plymouth, although after a severe accident in Lostwithiel in 1999 heavy goods vehicles are diverted from it at the top of Edgcumbe Road, Lostwithiel.