It is believed that resistive open faults can cause small delay defects at wires, contacts, and/or vias of a circuit. However, it remains to be elucidated whether any methods could diagnose resistive open faults. We propose a method for diagnosing resistive open faults by using a diagnostic delay fault simulation with the minimum detectable delay fault size. We also introduce a fault excitation function for the resistive open fault to improve the accuracy of the diagnostic result. The fault excitation function for the resistive open fault can determine a size of an additional delay at a faulty line with considering the effect of the adjacent lines. We demonstrated that the proposed method is capable of identifying fault locations for the resistive open fault with a small computation cost.