Abstract: A dynamic stiffness matrix method for the free vibration of steel-concrete composite beams is proposed based on the Timoshenko beam theory. In this method, the effects of interfacial shear slip, shear deformation and rotational inertia are considered. The results are exact because no approximate displacement and/or force fields are introduced in the element derivation. Compared with other Timoshenko composite beam models, the main advantage of the proposed method is that it assumes that each sub-beam has an independent rotary angle. This assumption is more consistent with the reality, leading to more accurate results. The eigenfrequencies obtained by the proposed method are compared with those by other methods in the literature using experimental models. The influence of shear deformation and rotational inertia on the frequency of composite beams with different shear connector stiffnesses and span-to-depth ratios is discussed in detail. The results show that the proposed method is more accurate than EBT and TBT with assumed identical rotary angles for the two sub-beams, especially for higher modes. Higher frequency, greater shear connector stiffness and smaller span-to-depth ratio result in larger relative errors for the Euler-Bernoulli beam theory. For the first, second and third frequencies, the errors of the Euler Bernoulli composite beam model are less than 5% when the span-to-depth ratio is greater than 10, 18 and 25, respectively.