All three emergency services were called to a collision between a truck and motorcycle at Altarnun, Launceston, on Sunday 9 August at about 10.15pm.
The arrested truck driver was a 21-year-old man from Bodmin.
The main arterial route through the county was closed westbound at Trewint Gap for almost seven hours.
Devon and Cornwall Police said the motorcyclist was pronounced dead at the scene by paramedics.
A pick-up truck driver was arrested on suspicion of causing death by dangerous driving after a male motorcyclist was killed on the A30 in Cornwall.