After one-step calcination of the as-prepared electrospun nanofibers, single crystalline YAG (Y3Al5O12) nanofibers were successfully synthesized in situ even when the diameter reached 800 nm. X-Ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) results confirmed that the calcined YAG nanofibers were pure body-centered cubic (bcc) YAG phase. The crystallography of single crystalline YAG nanofibers was perfect. After doping with Eu3+ ions, notable polarized photoluminescence (PL) was observed from an individual YAG: Eu3+ nanofiber, which was predominately owing to the dielectric mismatch between the nanofiber and its surroundings. The PL polarization ratio (rho) showed a strong diametrical dependence as a factor of similar to 1/R when the diameter (R) is comparable to the light wavelength. The large scale, super-long, single crystalline YAG nanofibers with notable polarized PL are promising to be applied as one dimensional nanofiber lasers, linearly polarized light resources, polarized sensors, photodetectors, etc.