We present the concept of an experimental method for the determination of the inertia matrix with an added mass effect. This experimental method can be applied to any size and type of object. In particular, it can be most effectively used for small unmanned aerial vehicles (UAVs). The model vehicle is suspended by two thin light weight low-stretch lines vertically attached to two hook points on the model and provided with a free rotational oscillation to estimate the moment of inertia around the vertical axis through the center of gravity. This procedure is repeated at different attitude angles so that the moments of inertia around different axes can be calculated and used in the estimation of all components of the moments and products of inertia in the form of an inertia matrix, using a multivariate regression equation.