To derive an analytical estimate of the lower eigenfrequency of a plane statically determinate truss, an inertial model of a truss with masses concentrated in the nodes of its lower chordis considered. Displacements of the nodes with masses are assumed to be vertical. The deflections of the truss under the action of concentrated forces applied to the nodes with masses and calculated by the Maxwell-Mohr formula give the values of the coefficients of the truss flexibility matrix. For the evaluation according to the method of Dunkerley only requires the diagonal elements of the matrix. The required estimate formula is obtained by induction calculation of the lower bound of the first frequency for individual trusses with a consistently increasing number of panels. This makes it possible to find the dependence of the frequency of oscillations of the truss not only on its size, but also on the number of panels. The coefficients of the formula are determined from the solution of recurrent equations for elements of sequences obtained from partial solutions. Maple computer mathematics system is used in calculations and analysis.