Abstract: The dynamic tests are carried out for 4 full scale models of cold-formed steel joists composite floors with different patterns of configuration. At a serviceability limit stage, effects to the fundamental frequency were studied, when the space among screws connect the roll-profiled steel sheet and the cold-formed steel joists was changed, or the rigid blocking, end constraints, floor live loads were different. By simulating the fundamental frequency of 16 different load cases using the finite element program based on ANSYS, the feasibility of the finite element method was approved. The factors which affect the fundamental frequency were analyzed through the finite element method, and a theoretical formula forecasting the fundamental frequency is then suggested. The research shows that the fundamental frequency declines with the increase of the live load of the floor, and that it can be improved by shortening the space among screws connecting the roll-profiled steel sheet and joists, by reducing the span depth ratio of floor joists or the width-thickness ratio of web, by setting rigid blockings at mid-span of adjacent joists, by strengthening the floor end constraints, and by increasing the thichness of joist or concrete slabs. The change of roll-profiled steel sheet shapes and the width of the floor bring little effect to the fundamental frequency. And the theoretical formula forecasting the fundamental frequency works well.