Abstract: This research investigated the slab effect on the stiffness, strength and energy dissipating of a composite joint with high performance steel. Four T-shaped beam-to-column joints were tested under the cyclic loading applied the beam end, which were made by Q390 steel. The experimental results indicate that the composite action of a concrete slab can significantly improve the stability of the beam top flange. The beam-to-column connection with an external-diaphragm is helpful to make a plastic hinge away from the connection and effectively avoid the occurrence of fracture in the vicinity of weld. However, owing to the slab effect, there is the possibility of fracture near the connection for this type connection. All hysteresis curves are stable, showing excellent energy dissipation capacity and ductility. The equivalent viscous damping coefficients of the specimens without slab are slightly higher than that with a concrete slab in the plastic point. And they tend to consistent when the story drift angle reaches 0.05 rad, showing that the combined effect of concrete slab has been ineffective.