The optical longitudinal differential protection theory for transmission lines at present is affected easily by distributed capacitance current and requires rigid synchronization of double-terminal information of lines. The zero-sequence differential impedance protection principle based on bilateral information is proposed. The definition of zero-sequence differential impedance and the criterion of zero-sequence impedance differential protection principle are given. The zero-sequence differential impedance is equal to the opposite value of the sum of both sides system zero sequence impedance, which is in the third quadrant, when the fault happens within the protection zero. The zero-sequence differential impedance is equal to the transmission line zero sequence impedance, which is in the first quadrant, when the fault happens out of the protection zero. Some models are built in PSCAD to verify the correctness of the zero sequence differential impedance protection principle. Experimental results show that the criterion is not influenced by the fault resistance, and can be applied in the line with or without shunt reactor and weak feeding system, and the strict synchronization of the sampled data is not needed. This work is supported by National Natural Science Foundation of China (No. 51377137).