Eucrite is one of the most important member of the HowarditeEucriteDiogenite (HED) meteorites and offers a unique opportunity to investigate thermal history of the Vestan crust. We studied the petrography and mineral chemistry of quartz and tridymite, the two major polymorphs of silica in basaltic Eucrites, aiming to investigate their genesis and its application to thermal history of Eucrites. Three weakly shocked unbrecciated samples (NWA 3162, NWA 6594, Igdi) and three heavily shocked monomict Eucrites (Millbillillie, Camel Donga, NWA 1654)were studied for evaluating the effect of shock reheating on the formation of quartz and tridymite. The results demonstrate that quartz and tridymite from all samples have similar petrographic, textural and geochemical characteristics respectively, whereas the texture of quartz varies among samples with different shock stages. These features, combined with thermal metamorphic history of Eucrites, reveal that quartz was not transformed from coexisting tridymite, but arose much earlier from preexisting hightemperature polymorph of SiO2. The tridymite now pervasively observed in Eucrites was the product of 〖JP2〗shockinduced reheating of quartz, followed by rapid cooling. Quartz and tridymite now presented in Eucrites, may have originated through the following processes: ① eruption of eucritic basaltic magma and crystallization of hightemperature polymorph 〖JP〗of SiO2. ② Transformation of quartz from hightemperature polymorph during intense thermal metamorphism. ③ Impactinduced partial melting of quartz and crystallization of tridymite, accompanied by development of hackle fracture in quartz in intenselyshocked samples. To summarize, quartz and tridymite now widely observed in Eucrites were products of different thermal events. The coexistence of quartz and tridymite in most Eucrites shows that they were disturbed after thermal metamorphism and there was pervasive local thermal heterogeneity induced by impact reheating. The results presented here also have important implications for selection of samples for insitu or wholerock isotopic dating, as well as for interpretation of in situ chronological data.