A handlebar assembly for a bicycle includes a handlebar tube, a sleeve, a clamping member and an end cap. Thus, the clamping plate of the end cap is clamped between any two of the clamping member, an end portion of the sleeve and an outer wall of the handlebar tube without obstructing the contact space between the clamping member and the outer wall of the handlebar tube so that the clamping member can directly clamp the handlebar tube completely or indirectly clamp the handlebar tube locally through the clamping plate of the end cap, thereby enhancing the clamping effect of the handlebar assembly.