  A model of global magnetic reconnection rate in relativistic collisionless
plasmas is developed and validated by the fully kinetic simulation. Through
considering the force balance at the upstream and downstream of the diffusion
region, we show that the global rate is bounded by a value $\sim 0.3$ even when
the local rate goes up to $\sim O(1)$ and the local inflow speed approaches the
speed of light in strongly magnetized plasmas. The derived model is general and
can be applied to magnetic reconnection under widely different circumstances.
