An error correction method improves measurement accuracy of a vector network analyzer by reducing reflection measurement errors for a broad class of devices, such as filters, switches, cables, couplers, attenuators, and other passive devices tested by vector network analyzers (VNAs) that are reciprocal, having a forward transmission coefficient S.sub.21 and a reverse transmission coefficient S.sub.12 that are equal. Errors due to impedance mismatches at the load port of a transmission/reflection (T/R) test set are corrected without impacting the measurement speed of the VNA. The source port of the T/R test set is calibrated and a reflection measurement is performed while an impedance matched thruline standard of known electrical length is coupled between the source port and load port of the T/R test set. The reflection measurement is corrected for the electrical length of the thruline standard to obtain a reflection measurement of the load port of the T/R test set. Then, the transmission and reflection characteristics of the DUT are measured. The reciprocity of the DUT and the reflection measurement of the load port are used to extract the actual input reflection coefficient of the DUT.