All REE deposits in the Mianning-Dechang REE ore belt of western Sichuan Province are closely related to syenite-carbonatite complex in genesis. Despite the similar geological backgrounds, syenite-carbonatite complexes have various occurrences. Also, various types of zircons in these syenites rather than carbonatite exhibit the forms of fresh zircons (type Ⅰ), recrystallization zircons (type Ⅱ), hydrothermal fluid altered zircons (type Ⅲ), and ore-forming fluids altered zircons (type Ⅳ). All these types of zircons were examined carefully by BSE images, CL images, Raman spectra, EMPA, and some other means. Type Ⅰ and type Ⅱ zircons have nearly identical crystal structures, chemical composition and BSE images, with edges of Type Ⅱ zircons being recrystallized and having well-developed crystal structure because of their smaller full-widths at half-maximum FWHM; type Ⅲ zircons have the murky transmissive images, destroyed sieve texture and more increased FWHM, some thorite inclusions and decreased Zr and relatively high content of U, Th, Hf, Ce; type Ⅳ zircons have experienced more serious alteration by ore-forming fluids with many thorite inclusions, more destroyed sieve texture and the most increased FWHM, and high concentrations of U, Th, Hf as well as La, Ce, Nd, Sm. Different zircons show various morphologies, inclusions, degrees of destroyed texture and isomorphous substitutions between U, Th, Hf, Ce and Zr, which, in turn, suggests the evolution from magmatic to hydrothermal processes in the formation of REE deposits.