A newly developed microprobe-based methodology (the Flank Method) for Fe³⁺/ΣFe quantification has been successfully applied to some natural garnets from the North Qilian eclogites by JEOL JXA-8100 microprobe at Peking University. The results demonstrated an obvious discrepancy in comparison with the outcomes by conventional stoichiometric calculations. This methodology allows to measure the Fe³⁺/ΣFe ratio and perform elemental analyses simultaneously in the same condition. Accurate in-situ measurement of Fe³⁺ content in garnet may bring certain impact on the garnet-based P-T estimation. According to the compositional zonation displayed in the studied eclogitic garnets from North Qilian, a prograde metamorphic PT path from 19.5 kbar, 520 °C to 22 kbar, 600 °C was reconstructed. More interestingly, the measured Fe³⁺/ΣFe ratios in garnets decreasing from core to rim may probably imply that the oxygen fugacity (fO₂) declines with the depth of the subduction zone.