Monodisperse rare earth (RE) fluoride colloidal nanocrystals (NCs) including REF₃ (RE = La, Pr, Nd), NaREF₄ (RE = Sm–Ho, Y) and Na₅RE₉F₃₂ (RE = Er, Yb, Lu) have been successfully synthesized by a facile one-step method using oleic acid as surfactant and 1-octadecene as solvent. The phase, morphology, size, and photoluminescence properties of as-synthesized NCs were well investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FT-IR), and photoluminescence (PL) spectra. The results reveal that the as-synthesized NCs consist of monodisperse colloidal NCs with narrow size distribution, which can easily disperse in non-polar cyclohexane solvent. The as-prepared NCs exhibit a rich variety of morphologies and different crystal phases (hexagonal or cubic), which may be related to the inherent natures of different rare earth ions. The possible formation mechanism of NCs with diverse architectures has been presented. In addition, representative Yb/Er, Yb/Tm, or Yb/Ho co-doped NaGdF₄ and Na₅Lu₉F₃₂ NCs exhibit intensive multicolor up-conversion (UC) luminescence under a single 980 nm NIR excitation, displaying potential applications in bioimaging and therapy. Moreover, transparent and UC fluorescent NCs–polydimethylsiloxane (PDMS) composites with regular dimensions were also prepared by an in situ polymerization route.