Altarnun is in the Hundred of Lesneweth, in the deanery of Trigg-major and is eight miles West of Launceston. It is surrounded by the parishes of Trewin, Lewannick and Northill on the East, St Cleer and St Neot on the South, Blisland and St Breward (Simon-Ward) on the West, and St Clether and Laneast on the North. The village is to the East of Bodmin Moor. It is not mentioned in the Domesday Book of 1086. Altarnun Parish is now in the North Cornwall district and part of it lies within Bodmin Moor. Part of the modern parish of Bolventor is in the South-West of the older Altarnun parish. The area is mainly moorland but there are areas of farmland to the East where animals graze the moorland areas. The River Lynher rises in this parish.