Zircon U-Pb geochronology has been widely used for determining the age of geologic processes. However, for very ancient zircon samples (e.g., ~3.8 Ga), the quality of U-Pb dating results may be affected by the relatively high-grade of Pb loss caused by the crystal damage through radioactive decay of U and Th. Here, we present a comparative study to explore the effect of thermal annealing (TA) and chemical abrasion (CA) on zircon U-Pb geochronology using zircon samples from some of the oldest igneous rocks on Earth (the Acasta Gneiss and the Muzidian Gneiss complexes), with the goal of getting better constrained zircon U-Pb ages of ancient rocks. Prior to the laser ablation dating experiments, all zircon samples and reference standards were thermally annealed (850 ℃ ~ 1050 ℃/48 h), and a subset of thermally annealed zircon samples was then selected for chemical abrasion treatment (partial dissolution using hydrofluoric acid at 170 ℃ for12 h), in order to remove radiation damaged domains. The results show that the U-Pb ages of CA zircon grains appear to be more concordant with smaller MSWDs than those of the TA and untreated zircon grains, indicating that the partial dissolution treatment of thermally annealed zircon grains effectively removed domains with Pb loss, leaving the rest of the zircon crystal lattice relatively unaffected. Therefore, for complex and ancient (> 3.8 Ga) zircon samples, chemical abrasion prior to laser ablation is shown to be useful for reducing Pb loss and data scatter. Thermal annealing appears to improve the matrix match between the unknown and reference zircon grains, which is especially useful for the dating of high U, young zircon samples. Additionally, our results show no systematic difference in the trace elements and Hf isotopic compositions between treated and untreated zircon samples, which demonstrates that the treatments do not have negative effect on the trace elements and Lu-Hf isotopic compositions in zircons.