It has been discovered in experiments recently that the dendritic integration of excitatory glutamatergic inputs and inhibitory GABAergic inputs obeys a simple arithmetic rule in pyramidal neurons.The somatic response can be well described by a linear sum of the excitatory postsynaptic potential (EPSP), the inhibitory postsynaptic potential (IPSP), and a nonlinear term proportional to their product (k*EPSP*IPSP), where the shunting coefficient k depends on the spatial location of excitatory input and inhibitory input.We demonstrate both theoretically and numerically that the above dendritic integration rule can be explained by sub-threshold membrane potential dynamics as characterized in the standard Integrate-and-Fire models.In order to account for spatial effects on the shunting coefficient k, we propose a simple modified Integrate-and-Fire model and our numerical results show that this model captures the phenomena as observed in experiments.