A portable computer with a rotary positioning structure is disclosed, which includes a first body, a second body, a shaft including a first part disposed on the first body and a second part disposed on the second body, a seat disposed to the first body, and a button. The second body is rotatable with the second part relative to the first part in a rotary path. The button is movable on the seat between a first position where the button is engaged into the rotary path to prohibit the second body from rotation, and a second position where the button is out of the rotary path for allowing rotation of the second body.