The invention relates to a method for installing an inertial reference unit in an aircraft, in which: a reference unit support (1) is fixed to a structural element of the aircraft; support installation angles—in heading, roll and pitch—representative of angular errors of positioning of the reference unit support relative to an aircraft coordinate system are determined, this determination being based on the measurement of coordinates of points of a previously defined reference coordinate system and of characteristic points of the reference unit support; the said support installation angles are recorded in memory means, for the purposes of subsequent correction of data measured by the inertial reference unit. Preferably the reference coordinate system used is defined by means of targets formed on the structure of a segment of the aircraft for the purposes of joining this segment with an adjacent segment.