The similarity transform and the properties of quaternion matrices are used to derive invariant expressions for the inertia matrix and kinetic energy of an asymmetric rigid body undergoing spatial motion around a fixed point in body-fixed and inertial frames of reference. The elements of the quaternion matrixes are Rodrigues-Hamilton parameters and quasi-velocities Introduction. The expediency of the invariant representation of the basic dynamic characteristics of an asymmetric rigid body in the algorithmization of mathematical simulation of nonlinear dynamics of complex mechanical systems in spatial motion was mentioned in [9]. The solution of this problem involves the principle of symmetry [10,16], Euler-Lagrange differential equations, Rodrigues-Hamilton, Cayley-Klein, and Euler parameters [13], quaternions [2, 3], Lush hypercomplex numbers [11], tensor calculus [4, 5], matrix calculus [15] and quaternion matrices [1,11,14]. These tendencies were reflected in some modern problems of aviation engineering, robotics, gyroscopy, vibroprotection [6,12,[18][19][20], where use is made of the matrix representation of mathematical models, which promotes the use of computer engineering.Here we use quaternion matrices for the invariant representation of the kinetic energy of an asymmetric rigid body undergoing spatial motion around a fixed point in the body-fixed and inertial frames of reference, which are applied in deriving the nonlinear equations of spatial motion in Euler-Lagrange symmetric matrix form [17]. Invariant representations are used in computer engineering to verify algorithms and control the precision of calculations [12].1. Problem Formulation. Kinetic energy is one of the main measures of mechanical motion (dynamic quantity). Kinetic energy underlies Lagrange's equations of the second kind for the generalized coordinates and the Euler-Lagrange equations for quasicoordinates [12]. The representation of the kinetic energy determines the structure of the differential equations of motion and analytic or numerical methods of solving them [7].The task is to represent the kinetic energy of an asymmetric rigid body undergoing spatial motion around an arbitrary fixed point in invariant, symmetric form for the inertial and body-fixed frames of reference. The invariant form is found using quaternion matrices, which also make symbolic and analytic transformations convenient. We choose two rectangular Cartesian frames of reference with origins at the fixed point. One frame OY Y Y 1 2 3 is rigidly fixed to the body, and the other frame OX X X 1 2 3 is inertial (Fig. 1).In determining the kinetic energy of a rigid body having a fixed point O, we proceed from the formula for the kinetic energy of a material particle dm [5]: