The paper investigates the effect of neutral axis locations of a composite section on the dynamic behaviour of composite floors. It is demonstrated that for a composite section the second moment of area of the section in respect to any assumed location of the neutral axis is always larger than the true value of the second moment of area of the section. The relative errors in the calculation of the second moment of a composite section between the actual location of the neutral axis and four other possible assumed locations of the neutral axis are examined. The calculated natural frequencies of several stiffened plates using the assumed locations of the neutral axis are compared with the available measured natural frequencies and numerical results. Finally, the effect of the assumed locations of the neutral axis on the dynamic characteristics of a real composite floor is assessed. It is concluded that the predicted natural frequencies are not sensitive to the assumed locations of the neutral axis, but disregard of the eccentricity in calculation will lead to underestimation, at least, of the fundamental natural frequency of a composite floor and will alter the orders of mode shapes.