The method to calculate principal axes of inertia by adjoint matrix of eigen ma-trix is given，examples are presented.

This paper presents the eigen equation of inertial tensor expressed by the three principal invariants, which is an algebra-equation solution to calculate eigen value;proves that eigenvalue is the principal moments of inertia and that eigenvector is the principal axes of inertia;and discusses a certain method to calculate prinpal axes of inertia by adjoint matrix of eigen matrix.

In rigid body mechanics, confirming the principal axis of inertia of rigid body is particularly important, its calculation can be made to be simplified and favorable to study the nature and movement law of rigid body.

The principal axis of inertia of a rigid body are its symmetric axis,the rigid body is possessed of geometrical symmetry and its mass are well distributed or its mass are symmetrially distributed.

The technology of the principal axis of inertia measurement mostly applies to the field of spaceflight and missile.

The inclination between the rotator s principal inertia axis and circumrotatory axis has been given with its couple imbalance value measured by the machine.

An algorithm for oblique license plate correction is proposed based on the corner point information and inertial principal axis.

In order to measure the inertial principal axis of projectile precisely to ensure the flight stability and hit probability,the two-facade four-point method of inertial principal axis was applied.